ci(msan): retry transient network failures while building the MSan image

test-infrastructure/Dockerfile.msan

@@ -16,18 +16,47 @@
 # Same pinned base as the primary test image — bump deliberately, never to a tag.
 FROM ubuntu:noble@sha256:4fbb8e6a8395de5a7550b33509421a2bafbc0aab6c06ba2cef9ebffbc7092d90
 
+# WHY every network step below is wrapped: on a cache miss the builder's
+# resolver hiccups and takes the whole lane down. PR-CI runs 33816056229 and
+# 33818108315 both died with `Could not resolve 'apt.llvm.org'` inside the
+# clang layer — seconds AFTER a wget from that same host had succeeded in the
+# same layer, i.e. a transient buildkit-side DNS failure, not a wrong URL.
+# retry: 5 attempts with growing backoff, and it still exits non-zero after
+# the last one, so a genuine breakage keeps failing the build.
+RUN printf '%s\n' \
+    '#!/bin/sh' \
+    '# retry <cmd> [args...]: 5 attempts, sleeping 5s/10s/20s/40s between them.' \
+    'attempt=1; delay=5' \
+    'until "$@"; do' \
+    '  if [ "$attempt" -ge 5 ]; then' \
+    '    echo "retry: giving up after $attempt attempts: $*" >&2' \
+    '    exit 1' \
+    '  fi' \
+    '  echo "retry: attempt $attempt failed, sleeping ${delay}s before retrying: $*" >&2' \
+    '  sleep "$delay"' \
+    '  attempt=$((attempt + 1)); delay=$((delay * 2))' \
+    'done' \
+    > /usr/local/bin/retry \
+    && chmod 0755 /usr/local/bin/retry
+
 # clang 22 from apt.llvm.org, matching the diag and analyzer lanes. Noble's
 # default is clang 18 — four majors behind everything else here, which is both
 # an inconsistency and a bad vantage point for debugging sanitizer behaviour.
-RUN apt-get update && apt-get install -y --no-install-recommends wget gnupg ca-certificates \
-    && wget -qO- https://apt.llvm.org/llvm-snapshot.gpg.key > /etc/apt/trusted.gpg.d/apt.llvm.org.asc \
+# The key is fetched with `wget -O <file>` rather than `-qO- > <file>`: under
+# retry a redirect is opened once for all attempts, so a partial write from a
+# failed attempt would be prepended to the output of a later successful one.
+RUN retry apt-get -o Acquire::Retries=3 update \
+    && retry apt-get -o Acquire::Retries=3 install -y --no-install-recommends wget gnupg ca-certificates \
+    && retry wget -q -O /etc/apt/trusted.gpg.d/apt.llvm.org.asc https://apt.llvm.org/llvm-snapshot.gpg.key \
     && echo "deb http://apt.llvm.org/noble/ llvm-toolchain-noble-22 main" > /etc/apt/sources.list.d/llvm-22.list \
-    && apt-get update && apt-get install -y --no-install-recommends \
+    && retry apt-get -o Acquire::Retries=3 update \
+    && retry apt-get -o Acquire::Retries=3 install -y --no-install-recommends \
        clang-22 libclang-rt-22-dev llvm-22 \
     && ln -sf /usr/bin/clang-22 /usr/bin/clang \
     && ln -sf /usr/bin/clang++-22 /usr/bin/clang++
 
-RUN apt-get update && apt-get install -y --no-install-recommends \
+RUN retry apt-get -o Acquire::Retries=3 update \
+    && retry apt-get -o Acquire::Retries=3 install -y --no-install-recommends \
     cmake \
     ninja-build \
     make \
@@ -40,9 +69,10 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     && rm -rf /var/lib/apt/lists/*
 
 # libc++ + libc++abi + libunwind with MemoryWithOrigins, at the SAME major as
-# the compiler above — a runtimes build must match its clang.
-RUN git clone --depth 1 --branch llvmorg-22.1.0 \
-        https://github.com/llvm/llvm-project.git /tmp/llvm-project \
+# the compiler above — a runtimes build must match its clang. The clone clears
+# its destination first: a half-finished clone would make every later attempt
+# fail on "destination path already exists".
+RUN retry sh -c 'rm -rf /tmp/llvm-project && git clone --depth 1 --branch llvmorg-22.1.0 https://github.com/llvm/llvm-project.git /tmp/llvm-project' \
     && cmake -G Ninja -S /tmp/llvm-project/runtimes -B /tmp/llvm-msan \
         -DCMAKE_BUILD_TYPE=Release \
         -DCMAKE_C_COMPILER=clang \
@@ -59,7 +89,7 @@ RUN git clone --depth 1 --branch llvmorg-22.1.0 \
     && rm -rf /tmp/llvm-project /tmp/llvm-msan
 
 # zlib with MSan (static, so the runner needs no runtime path for it).
-RUN git clone --depth 1 --branch v1.3.1 https://github.com/madler/zlib.git /tmp/zlib \
+RUN retry sh -c 'rm -rf /tmp/zlib && git clone --depth 1 --branch v1.3.1 https://github.com/madler/zlib.git /tmp/zlib' \
     && cd /tmp/zlib \
     && CC=clang CFLAGS="-fsanitize=memory -fsanitize-memory-track-origins=2 -fno-omit-frame-pointer -O2" \
        ./configure --prefix=/opt/msan --static \