[Fuzzing] Add a build config option which enables Clang source-based code coverage (#32858)

This maybe used to quickly verify the code coverage of a modified test
locally (e.g. fuzzer).

Example:
```
# Build and run target; the raw profile will be written to $LLVM_PROFILE_FILE when the program exits
$ bazel build --config=dbg --config=fuzzer_asan --config=coverage //test/core/end2end/fuzzers:api_fuzzer
$ LLVM_PROFILE_FILE="api_fuzzer.profraw" bazel-bin/test/core/end2end/fuzzers/api_fuzzer test/core/end2end/fuzzers/api_fuzzer_corpus/*

# Create coverage report
$ llvm-profdata-14 merge -sparse api_fuzzer.profraw -o api_fuzzer.profdata
$ llvm-cov-14 report ./bazel-bin/test/core/end2end/fuzzers/api_fuzzer --instr-profile=api_fuzzer.profdata
```

Sample report:
f94e444f25/gistfile1.txt

One trick is that the binary needs to be statically linked, e.g. specify
`linkstatic = 1` on the BUILD target.
See https://clang.llvm.org/docs/SourceBasedCodeCoverage.html for more
info.



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tools/bazel.rc

@@ -60,6 +60,10 @@ build:fuzzer_asan --linkopt=-fsanitize=fuzzer,address
 build:fuzzer_asan --action_env=ASAN_OPTIONS=detect_leaks=1:color=always
 build:fuzzer_asan --action_env=LSAN_OPTIONS=suppressions=test/core/util/lsan_suppressions.txt:report_objects=1
 
+build:coverage --copt=-fprofile-instr-generate
+build:coverage --copt=-fcoverage-mapping
+build:coverage --linkopt=-fprofile-instr-generate
+
 build:fork_support --cxxopt=-DGRPC_ENABLE_FORK_SUPPORT=1
 build:fork_support --cxxopt=-DGRPC_POSIX_FORK_ALLOW_PTHREAD_ATFORK=1
 build:fork_support --action_env=GRPC_ENABLE_FORK_SUPPORT=1

tools/fuzzing/generate_coverage_report.sh

@@ -0,0 +1,80 @@
+#!/bin/bash
+# Copyright 2023 The gRPC Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# This tool builds and runs a fuzzer target and generates coverage report under
+# /tmp.
+#
+# Example:
+# Run fuzzer with existing corpus (no fuzzing):
+# $ ./tools/fuzzing/generate_coverage_report.sh //test/core/end2end/fuzzers:api_fuzzer test/core/end2end/fuzzers/api_fuzzer_corpus/*
+#
+# Run with fuzzing:
+# $ ./tools/fuzzing/generate_coverage_report.sh //test/core/end2end/fuzzers:api_fuzzer -max_total_time=10 test/core/end2end/fuzzers/api_fuzzer_corpus
+#
+# Note that if a crash happened during fuzzing, the coverage data will not be dumped.
+# See https://github.com/google/fuzzing/issues/41#issuecomment-1027653690 for workaround.
+
+err() {
+  echo "$*" >&2
+}
+
+if [[ -z "$1" ]]; then
+  err "target not specified"
+  exit 1
+fi
+
+RANDOM=$(date +%s)
+FILENAME="${RANDOM}"
+
+export LLVM_PROFILE_FILE=/tmp/"${FILENAME}".profraw
+OUTPUT_BASE=$(bazel info output_base)
+MIDDLE="execroot/com_github_grpc_grpc/bazel-out/k8-dbg/bin"
+
+CLANG_MAJOR_VERSION=$(clang --version | grep version | sed -r 's/.*version ([^\.]+).*/\1/')
+LLVM_PROFDATA="llvm-profdata-${CLANG_MAJOR_VERSION}"
+LLVM_COV="llvm-cov-${CLANG_MAJOR_VERSION}"
+
+which "${LLVM_PROFDATA}"
+if (( $? != 0 )); then
+  err "${LLVM_PROFDATA} not found"
+  exit 1
+fi
+
+TARGET=$(bazel query "$1")
+TARGET_BINARY_PATH="${OUTPUT_BASE}/${MIDDLE}/$(echo ${TARGET:2} | sed 's/:/\//')"
+
+# Build:
+bazel build --dynamic_mode=off --config=dbg --config=fuzzer_asan --config=coverage "${TARGET}"
+# Run:
+"${TARGET_BINARY_PATH}" ${@:2}
+
+if [[ ! -e "${LLVM_PROFILE_FILE}" ]]; then
+  err "Profile file ${LLVM_PROFILE_FILE} not created"
+  exit 1
+fi
+
+# Create coverage report:
+"${LLVM_PROFDATA}" merge -sparse "${LLVM_PROFILE_FILE}" -o /tmp/"${FILENAME}".profdata
+"${LLVM_COV}" report "${TARGET_BINARY_PATH}" --format=text --instr-profile=/tmp/"${FILENAME}".profdata > /tmp/"${FILENAME}".cov
+
+if (( $? == 0 )); then
+  echo "Coverage summary report created: /tmp/${FILENAME}.cov"
+  echo "Merged profile data file:        /tmp/${FILENAME}.profdata"
+  echo "Raw profile data file:           /tmp/${FILENAME}.profraw"
+  echo "There are other ways to explore the data, see https://clang.llvm.org/docs/SourceBasedCodeCoverage.html#creating-coverage-reports"
+else
+  err "Something went wrong"
+  exit 1
+fi