Docker mode: TGZArchiver.fromFile() fails on jar/zip input

[stokpop]

Finding

When Execute(name, "/path/to/fat.jar") receives a regular file (not a directory), TGZArchiver.fromFile() assumes gzip+tar format:

// tgz_archiver.go:135
zr, err := gzip.NewReader(file)

This fails on jar/zip files with "gzip: invalid header".

CF vs Docker divergence

• CF mode (setup.go:337): calls cf push name -p source — CF CLI + bits service handles jar/zip extraction, so Execute(name, "/path/to/fat.jar") works.
• Docker mode (setup.go:105): calls archiver.Compress(path, source) which hits fromFile() — only gzip+tar is supported. Jars/zips fail.

Impact

Test authors who pass a jar file directly to Execute() get a passing test on CF but a broken test on Docker. The workaround is to pre-explode the jar into a directory before passing to Execute().

[ramonskie]

Investigation Results

Confirmed the issue. Traced the full call chain for both modes:

Root Cause

Docker mode (docker/setup.go:105 → TGZArchiver.Compress()):

• Compress() switch statement (tgz_archiver.go:51-58) routes regular files to fromFile()
• fromFile() unconditionally calls gzip.NewReader(file) on line 133
• .jar/.zip have magic bytes PK\x03\x04, not gzip → "gzip: invalid header" 💥

CF mode (cf/setup.go:337): cf push name -p source → CF CLI/bits-service handles upload natively. No archiver involved.

Why this exists

fromFile() was added in commit d1b7a95 (2022-06-09) specifically for re-compressing pre-built .tar.gz lifecycle/buildpack packages. It was never intended for general file support — the switch has no branch for non-gzip regular files.

Proposed Fix

Simple approach — no format guessing, no flags, no new API:

In Compress(), when input is a regular file:

1. Try gzip.NewReader() — if succeeds → use existing fromFile() (backward compat)
2. If gzip fails → new fromRegularFile() — wraps the file as a single tar entry with same uid/gid/prefix logic as fromDirectory()

Result: /tmp/app/fat.jar lands in staging container as-is. The buildpack decides what to do with it (detect, supply, etc.) — same as what CF mode provides.

Input	Before	After
.tar.gz	✅ works	✅ unchanged
directory	✅ works	✅ unchanged
fat.jar	❌ crash	✅ wrapped as single tar entry
source.zip	❌ crash	✅ wrapped (test authors pre-explode if needed)

No new dependencies — fix uses Go stdlib only. Happy to submit a PR if this direction looks right.