compile_native_go_fuzzer_v2: fix handling of multiple file matches

The script uses recursive grep to find fuzzer functions, which can match
function names in multiple files. 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.

Required for #14183

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 +23,25 @@ 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)"
 fi