cpp-httplib: fix broken build

[tc-agent]

Summary

The coverage build for cpp-httplib has been failing daily since 2026-04-29 (last successful coverage build: 2026-04-28). Fuzz Introspector currently reports code coverage as N/A.

Root cause

Upstream PR yhirose/cpp-httplib#2412 (commit 831b64b, 2026-03-28) added two new fuzz targets, url_parser_fuzzer and header_parser_fuzzer, to test/fuzzing/Makefile. Unlike the existing server_fuzzer and client_fuzzer rules, the new rules do not run zip -q -r ${target}_seed_corpus.zip corpus, so no *_seed_corpus.zip is produced for the new targets.

The breakage surfaced when #15426 (commit fa7217d, 2026-04-28) switched build.sh from make -j$(nproc) server_fuzzer to make -j$(nproc) all, which started compiling all four fuzzers and triggering corpus downloads for them. ClusterFuzz has no corpus backup for the two new targets, so the coverage runner ends up with an HTML/XML error response written to /corpus/${target}.zip, unzip fails, and the build aborts with Failed to unpack the corpus for url_parser_fuzzer. Providing ${target}_seed_corpus.zip in $OUT lets the runner fall back to seed corpora when the backup is missing.

Change

In build.sh, after make all, generate the missing seed-corpus zips for url_parser_fuzzer and header_parser_fuzzer, reusing the existing test/fuzzing/corpus/ directory in the same way the upstream Makefile does for server_fuzzer / client_fuzzer. The [ -f ... ] || guard skips the zip if the upstream Makefile starts producing one.

Coverage after fix

5-minute libfuzzer/address run from seed corpora:

fuzzer	line coverage
server_fuzzer	12.56% (1144 / 9106)
client_fuzzer	10.40% (920 / 8849)
header_parser_fuzzer	4.33% (390 / 9010)
url_parser_fuzzer	3.56% (321 / 9016)
merged	20.39% (1892 / 9280)

For prior-state comparison, Fuzz Introspector reports 23.85% line coverage on server_fuzzer from production fuzzing (last data 2026-04-28); the local 5-minute number is lower because the run is much shorter and starts only from the seed corpus.

[github-actions]

Fuzzing Coverage Report

Tested: project cpp-httplib · base c4ba43b → head 1fd175d · 300s total fuzz budget · updated 2026-05-28 04:08 UTC · workflow run

Metric	Before	After	Delta
Static reachability	49.2% (261/531)	49.2% (261/531)	+0.0%
Line coverage	21.1% (2004/9494)	22.7% (2124/9340)	+6.0%
Branch coverage	32.5% (825/2536)	31.3% (845/2698)	+2.4%
Function coverage	20.7% (189/913)	21.8% (199/913)	+5.3%

Per-harness

Harness	Lines before	Lines after	Δ
client_fuzzer	9.6% (866/9041)	10.6% (958/9041)	+10.6%
header_parser_fuzzer	5.5% (508/9204)	4.2% (390/9204)	-23.2%
server_fuzzer	11.5% (1075/9316)	12.2% (1134/9316)	+5.5%
url_parser_fuzzer	3.6% (330/9210)	2.4% (225/9210)	-31.8%

_{Δ = (after − before) / before, to accommodate that denominators may change. "new" when before is 0; "deleted" when after is 0.}