PR analyzer/109098 notes that the SARIF spec mandates that .sarif
files are UTF-8 encoded, but -fdiagnostics-format=sarif-file naively
assumes that the source files are UTF-8 encoded when quoting source
artefacts in the .sarif output, which can lead to us writing out
.sarif files with non-UTF-8 bytes in them (which break my reporting
scripts).

The root cause is that sarif_builder::maybe_make_artifact_content_object
was using maybe_read_file to load the file content as bytes, and
assuming they were UTF-8 encoded.

This patch reworks both overloads of this function (one used for the
whole file, the other for snippets of quoted lines) so that they go
through input.cc's file cache, which attempts to decode the input files
according to the input charset, and then encode as UTF-8.  They also
check that the result actually is UTF-8, for cases where the input
charset is missing, or incorrectly specified, and omit the quoted
source for such awkward cases.

Doing so fixes all of the cases I've encountered.

The patch adds a new:
  { dg-final { verify-sarif-file } }
directive to all SARIF test cases in the test suite, which verifies
that the output is UTF-8 encoded, and is valid JSON.  In particular
it verifies that when we complain about encoding problems, the .sarif
report we emit is itself correctly encoded.

gcc/ChangeLog:
	PR analyzer/109098
	* diagnostic-format-sarif.cc (read_until_eof): Delete.
	(maybe_read_file): Delete.
	(sarif_builder::maybe_make_artifact_content_object): Use
	get_source_file_content rather than maybe_read_file.
	Reject it if it's not valid UTF-8.
	* input.cc (file_cache_slot::get_full_file_content): New.
	(get_source_file_content): New.
	(selftest::check_cpp_valid_utf8_p): New.
	(selftest::test_cpp_valid_utf8_p): New.
	(selftest::input_cc_tests): Call selftest::test_cpp_valid_utf8_p.
	* input.h (get_source_file_content): New prototype.

gcc/testsuite/ChangeLog:
	PR analyzer/109098
	* c-c++-common/diagnostic-format-sarif-file-1.c: Add
	verify-sarif-file directive.
	* c-c++-common/diagnostic-format-sarif-file-2.c: Likewise.
	* c-c++-common/diagnostic-format-sarif-file-3.c: Likewise.
	* c-c++-common/diagnostic-format-sarif-file-4.c: Likewise.
	* c-c++-common/diagnostic-format-sarif-file-Wbidi-chars.c: New
	test case, adapted from Wbidi-chars-1.c.
	* c-c++-common/diagnostic-format-sarif-file-bad-utf8-pr109098-1.c:
	New test case.
	* c-c++-common/diagnostic-format-sarif-file-bad-utf8-pr109098-2.c:
	New test case.
	* c-c++-common/diagnostic-format-sarif-file-bad-utf8-pr109098-3.c:
	New test case, adapted from cpp/Winvalid-utf8-1.c.
	* c-c++-common/diagnostic-format-sarif-file-valid-CP850.c: New
	test case, adapted from gcc.dg/diagnostic-input-charset-1.c.
	* gcc.dg/plugin/crash-test-ice-sarif.c: Add verify-sarif-file
	directive.
	* gcc.dg/plugin/crash-test-write-though-null-sarif.c: Likewise.
	* gcc.dg/plugin/diagnostic-test-paths-5.c: Likewise.
	* lib/scansarif.exp (verify-sarif-file): New procedure.
	* lib/verify-sarif-file.py: New support script.

libcpp/ChangeLog:
	PR analyzer/109098
	* charset.cc (cpp_valid_utf8_p): New function.
	* include/cpplib.h (cpp_valid_utf8_p): New prototype.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>