fix: report a failed -o write instead of claiming success

[VXNCXNX]

I explicitly licence this contribution under the MIT licence.

What's broken

-o reports success after a write that failed, and exits 0.

$ scc --format json -o /nonexistentdir/z.json .
results written to /nonexistentdir/z.json
$ echo $?
0
$ ls /nonexistentdir/z.json
ls: cannot access '/nonexistentdir/z.json': No such file or directory

--format-multi has the same hole, exiting 0 with the message going to stdout:

$ scc --format-multi 'json:/nonexistentdir/m.json' . ; echo $?
0

The same mistakes on the sibling paths are already handled, which is what makes this look like an oversight:

$ scc --hotspots --depth 20 -o /nonexistentdir/x.json . ; echo $?
open /nonexistentdir/x.json: no such file or directory
1
$ scc --report=/nonexistentdir/r.html . ; echo $?
create report file: open /nonexistentdir/r.html: no such file or directory
1

Read-only directories and paths that are actually directories fail the same silent way. In a CI job that is a green build with no artifact.

The fix

processor.go had _ = os.WriteFile(...) followed unconditionally by the success line. It now checks the error, prints it to stderr and exits 1, following runReport a few hundred lines above, which does exactly this. formatters.go already built a message for the multi case but sent it to stdout and carried on, so it goes to stderr and exits too.

After:

$ scc --format json -o /nonexistentdir/z.json . ; echo $?
open /nonexistentdir/z.json: no such file or directory
1
$ scc --format-multi 'json:/nonexistentdir/m.json' . ; echo $?
/nonexistentdir/m.json unable to be written to for format json: open /nonexistentdir/m.json: no such file or directory
1

A successful write is unchanged, still results written to ... and exit 0.

os.Exit(1) rather than returning an error, because Process() is called from cobra's Run:, which has no error return, and the report path at processor.go:932 already resolves that the same way. Happy to plumb an error through instead if you would rather change the signature.

This changes an exit code, from 0 to 1, on a case that previously failed silently. That is the point of the change, but it is a visible difference for anyone whose script currently ignores it.

Verification

TestRegressionOutputWriteFailureExits in regression_test.go, one subtest per path. It observes the real exit code: runSCCDir runs the test binary as a subprocess, so os.Exit(1) is a genuine child-process exit. It asserts non-zero exit, absence of the success message, presence of the reason, and that no file was created. With both source changes reverted and the test kept, both subtests fail.

LANG=C go test ./... passes.

A note in case you see it too: on a machine with a comma-decimal LANG, four unrelated tests fail on master, because the formatters build their printer from os.Getenv("LANG") while the fixtures spell numbers with a dot. LANG=C is green before and after.

On issue #510

I looked at whether this explains #510 and I do not think it does. That reporter got the tabular summary written instead of JSON, which is the unknown-format fallback from a json2-less binary, matching your reply that the action needed updating. Their target directory was writable, so no write failed and nothing was swallowed. Same family of symptom, different cause.