Sccache crashes when run in parallel with /dev/null as the output

[emilio]

I was trying to reduce a test-case that doesn't compile with #545 + my-changes, but compiles with clang.

So I used creduce by comparing clang and sccache as follows:

#!/usr/bin/env bash                                                                                                                                                                                                                           

FILE=out.cpp
CLANG_COMMAND="/usr/bin/clang++ -x c++ -c $FILE -o /dev/null -DDEBUG=1 -DOS_POSIX=1 -DOS_LINUX=1 -DSTATIC_EXPORTABLE_JS_API -DMOZ_HAS_MOZGLUE -DMOZILLA_INTERNAL_API -DIMPL_LIBXUL -fPIC -DMOZILLA_CLIENT -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2 -fstack-protector-strong -fno-sized-deallocation -fno-aligned-new -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2 -fstack-protector-strong -fno-exceptions -fno-strict-aliasing -fno-rtti -fno-exceptions -fno-math-errno -pthread -pipe -g -Os -fno-omit-frame-pointer -funwind-tables -fdiagnostics-color=always"

$CLANG_COMMAND # >/dev/null 2>&1
CLANG_RESULT=$?

echo "clang: $CLANG_RESULT"
if [ $CLANG_RESULT -ne 0 ]; then
  exit 1
fi

sccache $CLANG_COMMAND # >/dev/null 2>&1
SCCACHE_RESULT=$?

echo "sccache: $SCCACHE_RESULT"
if [ $SCCACHE_RESULT -eq 0 ]; then
  exit 1
fi

exit 0

However, it seems like sccache, when run in parallel, may end up trying to create a temporary file in /dev/, and crashes like:

sccache: encountered fatal error
sccache: error : Permission denied (os error 13) at path "/dev/.tmpo4Y0uv"
sccache:  cause: Permission denied (os error 13) at path "/dev/.tmpo4Y0uv"

Workaround is using a different path from /dev/null of course, but it seems like this should work.

[sigiesec]

I remember a similar issue (unrelated to sccache) on some Azure nodes where clang wanted to write temporary files to an inaccessible directory. IIRC we needed to add the -save-temps option to fix that.

[luser]

FYI we had hit a similar issue with MSVC and worked around it by just making sccache refuse to cache compilations that output to nul on Windows: https://github.com/mozilla/sccache/blob/7e9daab24cb88802b2b97f37ae9d3816628bd90c/src/compiler/msvc.rs#L292