Fix compile_native_go_fuzzer_v2 to handle multiple file matches

The compile_native_go_fuzzer_v2 script uses recursive grep to find fuzzer
functions, which can match function names in multiple files (e.g., in
fuzz_test.go, corpus.go, and corpus_gen/main.go). This causes fuzzer_filename
to contain multiple files, breaking the convertLibFuzzerTestcaseToStdLibGo
command during coverage builds.

Fix by searching directly for the function signature with testing.F parameter,
which only exists in the actual fuzzer test file. This avoids false matches in
files that only reference the function name in comments or helper functions.
The script now also explicitly fails if multiple matching files are found.

This fix is needed for the Prometheus native fuzzing migration

Signed-off-by: Julien Pivotto <291750+roidelapluie@users.noreply.github.com>

Author: roidelapluie

infra/base-images/base-builder/compile_native_go_fuzzer_v2

@@ -23,17 +23,26 @@ tags="-tags gofuzz"
 # Get absolute path.
 abs_file_dir=$(go list $tags -f {{.Dir}} $path)
 
-# TODO(adamkorcz): Get rid of "-r" flag here.
-export fuzzer_filename=$(grep -r -l --include='*.go' -s "$function" "${abs_file_dir}")
+# Find the file containing the fuzzer function definition.
+# Search for the actual function signature with testing.F to avoid false matches
+# in files that only reference the function name (e.g., in comments or helper functions).
+export fuzzer_filename=$(grep -r -l --include='*.go' "func $function.*testing\.F" "${abs_file_dir}")
 
 # Import build_native_go_fuzzer
 SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 source "$SCRIPT_DIR/go_utils.sh"
 
-# Test if file contains a line with "func $function" and "testing.F".
-if [ $(grep -r "func $function" $fuzzer_filename | grep "testing.F" | wc -l) -eq 1 ]
+# Verify we found exactly one file with the fuzzer function.
+file_count=$(echo "$fuzzer_filename" | wc -w)
+if [ "$file_count" -eq 1 ] && [ -n "$fuzzer_filename" ] && [ -f "$fuzzer_filename" ]
 then
   build_native_go_fuzzer $fuzzer $function $abs_file_dir $path
+elif [ "$file_count" -gt 1 ]
+then
+  echo "Error: Found multiple files with func ${function}(f *testing.F):"
+  echo "$fuzzer_filename"
+  exit 1
 else
   echo "Could not find the function: func ${function}(f *testing.F)"
+  exit 1
 fi