[llvm-cov] Keep the detailed error message in CoverageMappingIterator

Currently errors returned from CoverageMappingIterator will have their
message part discarded. This commit copies that message when
constructing the new CoverageMapError object, and let it displayed at
the end caller (llvm-cov).

Related: llvm#65264

Author: whentojump

llvm/include/llvm/ProfileData/Coverage/CoverageMappingReader.h

@@ -45,6 +45,7 @@ class CoverageMappingIterator {
   CoverageMappingReader *Reader;
   CoverageMappingRecord Record;
   coveragemap_error ReadErr;
+  std::string ReadErrMsg;
 
   void increment();
 
@@ -80,15 +81,15 @@ class CoverageMappingIterator {
   }
   Expected<CoverageMappingRecord &> operator*() {
     if (ReadErr != coveragemap_error::success) {
-      auto E = make_error<CoverageMapError>(ReadErr);
+      auto E = make_error<CoverageMapError>(ReadErr, ReadErrMsg);
       ReadErr = coveragemap_error::success;
       return std::move(E);
     }
     return Record;
   }
   Expected<CoverageMappingRecord *> operator->() {
     if (ReadErr != coveragemap_error::success) {
-      auto E = make_error<CoverageMapError>(ReadErr);
+      auto E = make_error<CoverageMapError>(ReadErr, ReadErrMsg);
       ReadErr = coveragemap_error::success;
       return std::move(E);
     }

llvm/lib/ProfileData/Coverage/CoverageMappingReader.cpp

@@ -57,8 +57,10 @@ void CoverageMappingIterator::increment() {
     handleAllErrors(std::move(E), [&](const CoverageMapError &CME) {
       if (CME.get() == coveragemap_error::eof)
         *this = CoverageMappingIterator();
-      else
+      else {
         ReadErr = CME.get();
+        ReadErrMsg = CME.getMessage();
+      }
     });
 }
 

llvm/test/tools/llvm-cov/warnings.h

@@ -13,4 +13,4 @@
 // FAKE-FUNC-STDERR: Could not read coverage for '{{.*}}'.
 
 // RUN: not llvm-cov report %S/Inputs/malformedRegions.covmapping -instr-profile %S/Inputs/elf_binary_comdat.profdata 2>&1 | FileCheck %s -check-prefix=MALFORMED-REGION
-// MALFORMED-REGION: failed to load coverage: '{{.*}}malformedRegions.covmapping': malformed coverage data
+// MALFORMED-REGION: failed to load coverage: '{{.*}}malformedRegions.covmapping': malformed coverage data: counter mapping region locations are incorrect