scripts/download_frontend_libs.py

@@ -1,9 +1,12 @@
 #!/usr/bin/env python3
 from __future__ import annotations
 
+import os
 import re
+import time
 from pathlib import Path
 from urllib.parse import urljoin, urlparse
+from urllib.error import HTTPError, URLError
 from urllib.request import Request, urlopen
 
 ROOT = Path(__file__).resolve().parents[1]
@@ -31,6 +34,40 @@ GOOGLE_FONT_FAMILIES = (
     "Source Sans 3",
 )
 GOOGLE_FONT_WEIGHTS = "400;500;600;700;800"
+DOWNLOAD_RETRIES = int(os.environ.get("PYTORRENT_DOWNLOAD_RETRIES", "4"))
+DOWNLOAD_RETRY_DELAY = int(os.environ.get("PYTORRENT_DOWNLOAD_RETRY_DELAY", "10"))
+DOWNLOAD_TIMEOUT = int(os.environ.get("PYTORRENT_DOWNLOAD_TIMEOUT", "180"))
+
+
+def retry_countdown(seconds: int) -> None:
+    for remaining in range(seconds, 0, -1):
+        print(f"Retrying in {remaining}s...", end="\r", flush=True)
+        time.sleep(1)
+    if seconds > 0:
+        print(" " * 40, end="\r", flush=True)
+
+
+def candidate_urls(url: str) -> list[str]:
+    candidates = [url]
+    replacements = (
+        ("https://cdn.jsdelivr.net/npm/bootstrap@", "https://unpkg.com/bootstrap@"),
+        ("https://cdn.jsdelivr.net/npm/bootswatch@", "https://unpkg.com/bootswatch@"),
+        ("https://cdn.jsdelivr.net/npm/swagger-ui-dist@", "https://unpkg.com/swagger-ui-dist@"),
+        ("https://cdn.jsdelivr.net/gh/lipis/flag-icons@", "https://cdn.jsdelivr.net/npm/flag-icons@"),
+        ("https://cdn.jsdelivr.net/gh/DevExpress/bootstrap-themes@master/", "https://raw.githubusercontent.com/DevExpress/bootstrap-themes/master/"),
+        ("https://cdn.socket.io/", "https://cdnjs.cloudflare.com/ajax/libs/socket.io/"),
+        ("https://cdnjs.cloudflare.com/ajax/libs/font-awesome/", "https://cdn.jsdelivr.net/npm/@fortawesome/fontawesome-free@"),
+    )
+    for old, new in replacements:
+        if url.startswith(old):
+            candidates.append(url.replace(old, new, 1))
+    # font-awesome has a different path layout on npm/jsDelivr.
+    candidates = [item.replace("/css/all.min.css", "/css/all.min.css") for item in candidates]
+    unique = []
+    for item in candidates:
+        if item not in unique:
+            unique.append(item)
+    return unique
 
 
 def google_fonts_css_url() -> str:
@@ -147,15 +184,31 @@ def bootstrap_css_asset(theme: str) -> dict[str, str]:
 
 def download(url: str, dest: Path) -> None:
     dest.parent.mkdir(parents=True, exist_ok=True)
-    req = Request(url, headers={"User-Agent": "pyTorrent installer"})
-    with urlopen(req, timeout=60) as response:
-        data = response.read()
-    if not data:
-        raise RuntimeError(f"Empty response for {url}")
-    tmp = dest.with_suffix(dest.suffix + ".tmp")
-    tmp.write_bytes(data)
-    tmp.replace(dest)
-    print(f"OK {dest.relative_to(ROOT)}")
+    last_error: Exception | None = None
+    for candidate in candidate_urls(url):
+        for attempt in range(1, DOWNLOAD_RETRIES + 1):
+            try:
+                req = Request(candidate, headers={"User-Agent": "pyTorrent installer"})
+                with urlopen(req, timeout=DOWNLOAD_TIMEOUT) as response:
+                    data = response.read()
+                if not data:
+                    raise RuntimeError(f"Empty response for {candidate}")
+                tmp = dest.with_suffix(dest.suffix + ".tmp")
+                tmp.write_bytes(data)
+                tmp.replace(dest)
+                if candidate != url:
+                    print(f"OK {dest.relative_to(ROOT)} from fallback {candidate}")
+                else:
+                    print(f"OK {dest.relative_to(ROOT)}")
+                return
+            except (HTTPError, URLError, TimeoutError, OSError, RuntimeError) as exc:
+                last_error = exc
+                print(f"Download failed ({attempt}/{DOWNLOAD_RETRIES}) for {candidate}: {exc}")
+                if attempt < DOWNLOAD_RETRIES:
+                    retry_countdown(DOWNLOAD_RETRY_DELAY)
+        if candidate != candidate_urls(url)[-1]:
+            print(f"Trying alternative source: {candidate_urls(url)[candidate_urls(url).index(candidate) + 1]}")
+    raise RuntimeError(f"Failed to download {url}: {last_error}")
 
 
 def download_css_with_assets(url: str, dest: Path) -> None:
@@ -184,10 +237,22 @@ def download_google_fonts_css(url: str, dest: Path) -> None:
             "Accept": "text/css,*/*;q=0.1",
         },
     )
-    with urlopen(req, timeout=60) as response:
-        css = response.read().decode("utf-8", errors="ignore")
+    last_error: Exception | None = None
+    css = ""
+    for attempt in range(1, DOWNLOAD_RETRIES + 1):
+        try:
+            with urlopen(req, timeout=DOWNLOAD_TIMEOUT) as response:
+                css = response.read().decode("utf-8", errors="ignore")
+            if not css.strip():
+                raise RuntimeError(f"Empty response for {url}")
+            break
+        except (HTTPError, URLError, TimeoutError, OSError, RuntimeError) as exc:
+            last_error = exc
+            print(f"Download failed ({attempt}/{DOWNLOAD_RETRIES}) for {url}: {exc}")
+            if attempt < DOWNLOAD_RETRIES:
+                retry_countdown(DOWNLOAD_RETRY_DELAY)
     if not css.strip():
-        raise RuntimeError(f"Empty response for {url}")
+        raise RuntimeError(f"Failed to download {url}: {last_error}")
 
     def replace_url(match: re.Match[str]) -> str:
         quote = match.group(1) or ""

scripts/install_pytorrent.sh

@@ -24,6 +24,10 @@ REPO_URL="${PYTORRENT_REPO_URL:-https://github.com/zdzichu6969/pyTorrent}"
 REPO_BRANCH="${PYTORRENT_REPO_BRANCH:-master}"
 WORK_DIR="${PYTORRENT_BOOTSTRAP_DIR:-/tmp/pytorrent-only-installer}"
 KEEP_WORK_DIR="${PYTORRENT_KEEP_BOOTSTRAP_DIR:-0}"
+DOWNLOAD_RETRIES="${PYTORRENT_DOWNLOAD_RETRIES:-4}"
+DOWNLOAD_RETRY_DELAY="${PYTORRENT_DOWNLOAD_RETRY_DELAY:-10}"
+DOWNLOAD_CONNECT_TIMEOUT="${PYTORRENT_DOWNLOAD_CONNECT_TIMEOUT:-30}"
+DOWNLOAD_MAX_TIME="${PYTORRENT_DOWNLOAD_MAX_TIME:-600}"
 
 default_archive_url() {
     case "${REPO_URL%/}" in
@@ -61,13 +65,65 @@ prepare_downloader() {
     fail "curl or wget is required."
 }
 
+retry_countdown() {
+    local seconds="$1"
+    local remaining
+    for ((remaining=seconds; remaining>0; remaining--)); do
+        printf 'Retrying in %ss...\r' "${remaining}"
+        sleep 1
+    done
+    [[ "${seconds}" -gt 0 ]] && printf '%*s\r' 40 ''
+}
+
+archive_url_candidates() {
+    local url="$1"
+    printf '%s\n' "${url}"
+    case "${url}" in
+        https://github.com/*/archive/refs/heads/*.tar.gz)
+            local rest owner repo branch
+            rest="${url#https://github.com/}"
+            owner="${rest%%/*}"
+            rest="${rest#*/}"
+            repo="${rest%%/*}"
+            branch="${url##*/}"
+            branch="${branch%.tar.gz}"
+            printf 'https://codeload.github.com/%s/%s/tar.gz/refs/heads/%s\n' "${owner}" "${repo}" "${branch}"
+            ;;
+        https://github.com/*/archive/*.tar.gz)
+            local rest owner repo ref
+            rest="${url#https://github.com/}"
+            owner="${rest%%/*}"
+            rest="${rest#*/}"
+            repo="${rest%%/*}"
+            ref="${url##*/}"
+            ref="${ref%.tar.gz}"
+            printf 'https://codeload.github.com/%s/%s/tar.gz/%s\n' "${owner}" "${repo}" "${ref}"
+            ;;
+    esac
+}
+
 download_file() {
-    local url="$1" destination="$2"
-    if [[ "${DOWNLOADER}" == "curl" ]]; then
-        curl -fL "${url}" -o "${destination}"
-    else
-        wget -O "${destination}" "${url}"
-    fi
+    local url="$1"
+    local destination="$2"
+    local candidate attempt status
+    while IFS= read -r candidate; do
+        [[ -n "${candidate}" ]] || continue
+        for ((attempt=1; attempt<=DOWNLOAD_RETRIES; attempt++)); do
+            if [[ "${DOWNLOADER}" == "curl" ]]; then
+                curl -fL --connect-timeout "${DOWNLOAD_CONNECT_TIMEOUT}" --max-time "${DOWNLOAD_MAX_TIME}" "${candidate}" -o "${destination}" && return 0
+                status=$?
+            else
+                wget --timeout="${DOWNLOAD_CONNECT_TIMEOUT}" --read-timeout="${DOWNLOAD_MAX_TIME}" --tries=1 -O "${destination}" "${candidate}" && return 0
+                status=$?
+            fi
+            log "Download failed (${attempt}/${DOWNLOAD_RETRIES}) from ${candidate} (exit ${status})."
+            if [[ "${attempt}" -lt "${DOWNLOAD_RETRIES}" ]]; then
+                retry_countdown "${DOWNLOAD_RETRY_DELAY}"
+            fi
+        done
+        log "Trying alternative source if available after: ${candidate}"
+    done < <(archive_url_candidates "${url}")
+    return 1
 }
 
 cleanup() {

scripts/install_stack.sh

@@ -17,6 +17,10 @@ REPO_URL="${PYTORRENT_REPO_URL:-https://github.com/zdzichu6969/pyTorrent}"
 REPO_BRANCH="${PYTORRENT_REPO_BRANCH:-master}"
 WORK_DIR="${PYTORRENT_BOOTSTRAP_DIR:-/tmp/pytorrent-stack-installer}"
 KEEP_WORK_DIR="${PYTORRENT_KEEP_BOOTSTRAP_DIR:-0}"
+DOWNLOAD_RETRIES="${PYTORRENT_DOWNLOAD_RETRIES:-4}"
+DOWNLOAD_RETRY_DELAY="${PYTORRENT_DOWNLOAD_RETRY_DELAY:-10}"
+DOWNLOAD_CONNECT_TIMEOUT="${PYTORRENT_DOWNLOAD_CONNECT_TIMEOUT:-30}"
+DOWNLOAD_MAX_TIME="${PYTORRENT_DOWNLOAD_MAX_TIME:-600}"
 
 default_archive_url() {
     case "${REPO_URL%/}" in
@@ -105,14 +109,65 @@ prepare_downloader() {
     fail "curl or wget is required and no supported package manager was found."
 }
 
+retry_countdown() {
+    local seconds="$1"
+    local remaining
+    for ((remaining=seconds; remaining>0; remaining--)); do
+        printf 'Retrying in %ss...\r' "${remaining}"
+        sleep 1
+    done
+    [[ "${seconds}" -gt 0 ]] && printf '%*s\r' 40 ''
+}
+
+archive_url_candidates() {
+    local url="$1"
+    printf '%s\n' "${url}"
+    case "${url}" in
+        https://github.com/*/archive/refs/heads/*.tar.gz)
+            local rest owner repo branch
+            rest="${url#https://github.com/}"
+            owner="${rest%%/*}"
+            rest="${rest#*/}"
+            repo="${rest%%/*}"
+            branch="${url##*/}"
+            branch="${branch%.tar.gz}"
+            printf 'https://codeload.github.com/%s/%s/tar.gz/refs/heads/%s\n' "${owner}" "${repo}" "${branch}"
+            ;;
+        https://github.com/*/archive/*.tar.gz)
+            local rest owner repo ref
+            rest="${url#https://github.com/}"
+            owner="${rest%%/*}"
+            rest="${rest#*/}"
+            repo="${rest%%/*}"
+            ref="${url##*/}"
+            ref="${ref%.tar.gz}"
+            printf 'https://codeload.github.com/%s/%s/tar.gz/%s\n' "${owner}" "${repo}" "${ref}"
+            ;;
+    esac
+}
+
 download_file() {
     local url="$1"
     local destination="$2"
-    if [[ "${DOWNLOADER}" == "curl" ]]; then
-        curl -fL "${url}" -o "${destination}"
-    else
-        wget -O "${destination}" "${url}"
-    fi
+    local candidate attempt status
+    while IFS= read -r candidate; do
+        [[ -n "${candidate}" ]] || continue
+        for ((attempt=1; attempt<=DOWNLOAD_RETRIES; attempt++)); do
+            if [[ "${DOWNLOADER}" == "curl" ]]; then
+                curl -fL --connect-timeout "${DOWNLOAD_CONNECT_TIMEOUT}" --max-time "${DOWNLOAD_MAX_TIME}" "${candidate}" -o "${destination}" && return 0
+                status=$?
+            else
+                wget --timeout="${DOWNLOAD_CONNECT_TIMEOUT}" --read-timeout="${DOWNLOAD_MAX_TIME}" --tries=1 -O "${destination}" "${candidate}" && return 0
+                status=$?
+            fi
+            log "Download failed (${attempt}/${DOWNLOAD_RETRIES}) from ${candidate} (exit ${status})."
+            if [[ "${attempt}" -lt "${DOWNLOAD_RETRIES}" ]]; then
+                retry_countdown "${DOWNLOAD_RETRY_DELAY}"
+            fi
+        done
+        log "Trying alternative source if available after: ${candidate}"
+    done < <(archive_url_candidates "${url}")
+    return 1
 }
 
 detect_os_family() {

scripts/stack_installers/install_rtorrent.py

@@ -24,6 +24,54 @@ DEFAULT_CURL_REF = "8.19.0"
 DEFAULT_SERVICE_PATH = "/etc/systemd/system/rtorrent@.service"
 DEFAULT_SCGI_PORT = 5000
 DEFAULT_TORRENT_PORT = 51300
+DOWNLOAD_RETRIES = int(os.environ.get("PYTORRENT_DOWNLOAD_RETRIES", "4"))
+DOWNLOAD_RETRY_DELAY = int(os.environ.get("PYTORRENT_DOWNLOAD_RETRY_DELAY", "10"))
+DOWNLOAD_CONNECT_TIMEOUT = int(os.environ.get("PYTORRENT_DOWNLOAD_CONNECT_TIMEOUT", "30"))
+DOWNLOAD_MAX_TIME = int(os.environ.get("PYTORRENT_DOWNLOAD_MAX_TIME", "600"))
+
+
+def retry_countdown(seconds):
+    for remaining in range(seconds, 0, -1):
+        print(f"Retrying in {remaining}s...", end="\r", flush=True)
+        time.sleep(1)
+    if seconds > 0:
+        print(" " * 40, end="\r", flush=True)
+
+
+def run_with_retry(cmd, *, retries=DOWNLOAD_RETRIES, retry_delay=DOWNLOAD_RETRY_DELAY, retry_label=None, **kwargs):
+    last_error = None
+    label = retry_label or " ".join(str(x) for x in cmd[:3])
+    for attempt in range(1, retries + 1):
+        try:
+            return run(cmd, **kwargs)
+        except InstallError as exc:
+            last_error = exc
+            print(f"{label} failed ({attempt}/{retries}): {exc}")
+            if attempt < retries:
+                retry_countdown(retry_delay)
+    raise last_error
+
+
+def download_url_candidates(url):
+    candidates = [url]
+    if url.startswith("https://github.com/c-ares/c-ares/releases/download/v") and url.endswith(".tar.gz"):
+        version = url.rsplit("/c-ares-", 1)[-1].removesuffix(".tar.gz")
+        candidates.append(f"https://codeload.github.com/c-ares/c-ares/tar.gz/refs/tags/v{version}")
+    if url.startswith("https://curl.se/download/curl-") and url.endswith(".tar.gz"):
+        version = url.rsplit("/curl-", 1)[-1].removesuffix(".tar.gz")
+        tag = "curl-" + version.replace(".", "_")
+        candidates.append(f"https://github.com/curl/curl/releases/download/{tag}/curl-{version}.tar.gz")
+        candidates.append(f"https://codeload.github.com/curl/curl/tar.gz/refs/tags/{tag}")
+    if "sourceforge.net/projects/xmlrpc-c/files/latest/download" in url:
+        candidates.append("https://downloads.sourceforge.net/project/xmlrpc-c/latest/download")
+    if url.startswith("https://downloads.sourceforge.net/project/xmlrpc-c/"):
+        candidates.append(url.replace("https://downloads.sourceforge.net/", "https://sourceforge.net/projects/").replace("project/xmlrpc-c/", "xmlrpc-c/files/"))
+
+    unique = []
+    for candidate in candidates:
+        if candidate not in unique:
+            unique.append(candidate)
+    return unique
 
 
 class InstallError(Exception):
@@ -220,17 +268,35 @@ def clone_or_update_repo(repo_url, repo_dir, ref, *, debug=False):
     repo_dir = Path(repo_dir)
     if not repo_dir.exists():
         with Spinner(f"Cloning {repo_dir.name}", enabled=not debug):
-            run(["git", "clone", repo_url, str(repo_dir)], debug=debug)
+            run_with_retry(["git", "clone", repo_url, str(repo_dir)], debug=debug, retry_label=f"git clone {repo_url}")
     else:
         print(f"Repository already exists: {repo_dir}")
     with Spinner(f"Checking out {repo_dir.name} -> {ref}", enabled=not debug):
-        run(["git", "fetch", "--all", "--tags"], cwd=str(repo_dir), debug=debug)
+        run_with_retry(["git", "fetch", "--all", "--tags"], cwd=str(repo_dir), debug=debug, retry_label=f"git fetch {repo_dir.name}")
         run(["git", "checkout", ref], cwd=str(repo_dir), debug=debug)
-        run(["git", "pull", "--ff-only"], cwd=str(repo_dir), check=False, debug=debug)
+        run_with_retry(["git", "pull", "--ff-only"], cwd=str(repo_dir), check=False, debug=debug, retry_label=f"git pull {repo_dir.name}")
 
 
 def download_file(url, destination, *, debug=False):
-    run(["curl", "-fL", url, "-o", str(destination)], debug=debug)
+    last_error = None
+    for candidate in download_url_candidates(url):
+        for attempt in range(1, DOWNLOAD_RETRIES + 1):
+            try:
+                return run([
+                    "curl",
+                    "-fL",
+                    "--connect-timeout", str(DOWNLOAD_CONNECT_TIMEOUT),
+                    "--max-time", str(DOWNLOAD_MAX_TIME),
+                    candidate,
+                    "-o", str(destination),
+                ], debug=debug)
+            except InstallError as exc:
+                last_error = exc
+                print(f"Download failed ({attempt}/{DOWNLOAD_RETRIES}) from {candidate}: {exc}")
+                if attempt < DOWNLOAD_RETRIES:
+                    retry_countdown(DOWNLOAD_RETRY_DELAY)
+        print(f"Trying alternative source if available after: {candidate}")
+    raise last_error or InstallError(f"Download failed: {url}")
 
 
 def extract_tarball(tarball, destination, *, debug=False):

scripts/stack_installers/install_rtorrent_rhel.py

@@ -24,6 +24,54 @@ DEFAULT_CURL_REF = "8.19.0"
 DEFAULT_SERVICE_PATH = "/etc/systemd/system/rtorrent@.service"
 DEFAULT_SCGI_PORT = 5000
 DEFAULT_TORRENT_PORT = 51300
+DOWNLOAD_RETRIES = int(os.environ.get("PYTORRENT_DOWNLOAD_RETRIES", "4"))
+DOWNLOAD_RETRY_DELAY = int(os.environ.get("PYTORRENT_DOWNLOAD_RETRY_DELAY", "10"))
+DOWNLOAD_CONNECT_TIMEOUT = int(os.environ.get("PYTORRENT_DOWNLOAD_CONNECT_TIMEOUT", "30"))
+DOWNLOAD_MAX_TIME = int(os.environ.get("PYTORRENT_DOWNLOAD_MAX_TIME", "600"))
+
+
+def retry_countdown(seconds):
+    for remaining in range(seconds, 0, -1):
+        print(f"Retrying in {remaining}s...", end="\r", flush=True)
+        time.sleep(1)
+    if seconds > 0:
+        print(" " * 40, end="\r", flush=True)
+
+
+def run_with_retry(cmd, *, retries=DOWNLOAD_RETRIES, retry_delay=DOWNLOAD_RETRY_DELAY, retry_label=None, **kwargs):
+    last_error = None
+    label = retry_label or " ".join(str(x) for x in cmd[:3])
+    for attempt in range(1, retries + 1):
+        try:
+            return run(cmd, **kwargs)
+        except InstallError as exc:
+            last_error = exc
+            print(f"{label} failed ({attempt}/{retries}): {exc}")
+            if attempt < retries:
+                retry_countdown(retry_delay)
+    raise last_error
+
+
+def download_url_candidates(url):
+    candidates = [url]
+    if url.startswith("https://github.com/c-ares/c-ares/releases/download/v") and url.endswith(".tar.gz"):
+        version = url.rsplit("/c-ares-", 1)[-1].removesuffix(".tar.gz")
+        candidates.append(f"https://codeload.github.com/c-ares/c-ares/tar.gz/refs/tags/v{version}")
+    if url.startswith("https://curl.se/download/curl-") and url.endswith(".tar.gz"):
+        version = url.rsplit("/curl-", 1)[-1].removesuffix(".tar.gz")
+        tag = "curl-" + version.replace(".", "_")
+        candidates.append(f"https://github.com/curl/curl/releases/download/{tag}/curl-{version}.tar.gz")
+        candidates.append(f"https://codeload.github.com/curl/curl/tar.gz/refs/tags/{tag}")
+    if "sourceforge.net/projects/xmlrpc-c/files/latest/download" in url:
+        candidates.append("https://downloads.sourceforge.net/project/xmlrpc-c/latest/download")
+    if url.startswith("https://downloads.sourceforge.net/project/xmlrpc-c/"):
+        candidates.append(url.replace("https://downloads.sourceforge.net/", "https://sourceforge.net/projects/").replace("project/xmlrpc-c/", "xmlrpc-c/files/"))
+
+    unique = []
+    for candidate in candidates:
+        if candidate not in unique:
+            unique.append(candidate)
+    return unique
 
 
 class InstallError(Exception):
@@ -221,17 +269,35 @@ def clone_or_update_repo(repo_url, repo_dir, ref, *, debug=False):
     repo_dir = Path(repo_dir)
     if not repo_dir.exists():
         with Spinner(f"Cloning {repo_dir.name}", enabled=not debug):
-            run(["git", "clone", repo_url, str(repo_dir)], debug=debug)
+            run_with_retry(["git", "clone", repo_url, str(repo_dir)], debug=debug, retry_label=f"git clone {repo_url}")
     else:
         print(f"Repository already exists: {repo_dir}")
     with Spinner(f"Checking out {repo_dir.name} -> {ref}", enabled=not debug):
-        run(["git", "fetch", "--all", "--tags"], cwd=str(repo_dir), debug=debug)
+        run_with_retry(["git", "fetch", "--all", "--tags"], cwd=str(repo_dir), debug=debug, retry_label=f"git fetch {repo_dir.name}")
         run(["git", "checkout", ref], cwd=str(repo_dir), debug=debug)
-        run(["git", "pull", "--ff-only"], cwd=str(repo_dir), check=False, debug=debug)
+        run_with_retry(["git", "pull", "--ff-only"], cwd=str(repo_dir), check=False, debug=debug, retry_label=f"git pull {repo_dir.name}")
 
 
 def download_file(url, destination, *, debug=False):
-    run(["curl", "-fL", url, "-o", str(destination)], debug=debug)
+    last_error = None
+    for candidate in download_url_candidates(url):
+        for attempt in range(1, DOWNLOAD_RETRIES + 1):
+            try:
+                return run([
+                    "curl",
+                    "-fL",
+                    "--connect-timeout", str(DOWNLOAD_CONNECT_TIMEOUT),
+                    "--max-time", str(DOWNLOAD_MAX_TIME),
+                    candidate,
+                    "-o", str(destination),
+                ], debug=debug)
+            except InstallError as exc:
+                last_error = exc
+                print(f"Download failed ({attempt}/{DOWNLOAD_RETRIES}) from {candidate}: {exc}")
+                if attempt < DOWNLOAD_RETRIES:
+                    retry_countdown(DOWNLOAD_RETRY_DELAY)
+        print(f"Trying alternative source if available after: {candidate}")
+    raise last_error or InstallError(f"Download failed: {url}")
 
 
 def extract_tarball(tarball, destination, *, debug=False):