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Wrapper recursion: why filter Bear's lookup PATH, not the alternatives

docs/rationale/wrapper-recursion-ccache-alternatives.md

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Wrapper recursion: why filter Bear's lookup PATH, not the alternatives

Context

In wrapper mode, a compiler env var or PATH discovery can resolve to a masquerade wrapper (ccache, distcc, icecc, ...) rather than the real compiler. If Bear records that path, the build loops the wrapper into itself. Several fixes were considered before settling on "resolve past the masquerade dir and record the real compiler".

How the loop arises: masquerade wrappers (ccache, distcc, icecream, colorgcc, buildcache) install a directory of symlinks named after real compilers and prepend it to PATH; a bare gcc then resolves to the wrapper, which looks up the real compiler further along PATH (skipping its own symlinks) and forwards the call. Bear's wrapper mode prepends its own directory of compiler-named links to PATH as well. A wrapper that recognises itself only by symlink comparison cannot tell Bear's link from a real compiler, selects it as the "real" gcc, and the two forward to each other without end. distcc avoids that specific loop by stripping every PATH entry up to and including its own directory, but that also drops Bear's directory, silently breaking nested interception.

Decision

Filter Bear's own lookup PATH so resolution lands on the real compiler past any masquerade directory, and record that path. Do not touch the child's PATH or set ccache-specific variables.

Consequences

Detecting a masquerade wrapper by resolving the symlink of the discovered path, rather than matching a list of known directory locations, covers distribution-local masquerade setups automatically: any installer that symlinks compiler names to a wrapper binary is caught, with no per-distro path table to maintain. The cost is missing a masquerade installed as a shell script or hard copy, which no major distro ships.

The rejected alternatives were each narrower or more fragile:

  • Set CCACHE_COMPILER in the wrapper's child environment (the original proposal). The path the wrapper knows IS the ccache symlink (that is what which gcc returned at setup), and CCACHE_COMPILER pointing at a symlink-to-ccache makes ccache recurse into itself. Empirically verified on Fedora: CCACHE_COMPILER=/usr/lib64/ccache/gcc ccache gcc -c foo.c hangs and must be killed; CCACHE_COMPILER=/usr/bin/gcc works. Fixing it would require resolving past ccache to get the real path anyway - which is what this requirement does, making CCACHE_COMPILER redundant. It is also ccache-specific and would not help icecc or distcc.
  • CCACHE_PATH set to PATH minus .bear/. ccache-specific (no equivalent for other wrappers), still requires enumerating a safe PATH, and does not address Bear's config pointing at the wrong executable.
  • Removing masquerade directories from the child's PATH. Heavy- handed: a masquerade dir might also hold binaries that do not loop (some installs put distcc itself there); stripping it globally would break those. Filtering Bear's own lookup PATH is the narrower intervention.

References