covmerge

Overview ¶

Command covmerge merges Go coverage profiles (as produced by `go test -covermode=atomic -coverprofile=...`) into a single deduplicated profile.

Every unit/integration/e2e CI job in this repo runs with a wide -coverpkg (all instrumented packages), so each profile it emits already contains a coverage block for every package under test -- including ones that specific job never exercised (recorded with a zero count). Naively concatenating profiles (`tail -n +2 "$f" >> coverage.out` for each file, as a plain `cat`-style merge would) therefore duplicates every block once per job. With ~6 jobs and ~200 covered packages that inflates the merged file enough to blow past Node's ~512MB string length limit when a downstream step reads it back with fs.readFileSync.

covmerge instead parses each line's block key -- everything before the trailing execution count -- and sums counts for duplicate keys, emitting exactly one line per block. This is the same merge semantics as the well-known gocovmerge tool, specialized to atomic/count-style profiles (the only mode this repo's CI produces) and implemented with the standard library only so it needs no extra go.mod dependency.

Profiles are read and merged line-by-line via bufio.Scanner, so memory use is proportional to the number of distinct coverage blocks in the codebase, not to the combined size of the input files.

Usage:

go run ./cmd/covmerge -o coverage.out unit-0-coverage.out unit-1-coverage.out ...

With no -o flag, the merged profile is written to stdout.