[Source maps] Handle single-segment entries in source map header decoder (#6794)

Single-segment mappings were already handled in readNextDebugLocation,
but not in readSourceMapHeader.

Author: osa1

src/wasm/wasm-binary.cpp

@@ -2941,13 +2941,20 @@ void WasmBinaryReader::readSourceMapHeader() {
   //       investigation (if it does, it will assert in readBase64VLQ, so it
   //       would not be a silent error at least).
   uint32_t position = readBase64VLQ(*sourceMap);
-  uint32_t fileIndex = readBase64VLQ(*sourceMap);
-  uint32_t lineNumber =
-    readBase64VLQ(*sourceMap) + 1; // adjust zero-based line number
-  uint32_t columnNumber = readBase64VLQ(*sourceMap);
   nextDebugPos = position;
-  nextDebugLocation = {fileIndex, lineNumber, columnNumber};
-  nextDebugLocationHasDebugInfo = true;
+
+  auto peek = sourceMap->peek();
+  if (peek == ',' || peek == '\"') {
+    // This is a 1-length entry, so the next location has no debug info.
+    nextDebugLocationHasDebugInfo = false;
+  } else {
+    uint32_t fileIndex = readBase64VLQ(*sourceMap);
+    uint32_t lineNumber =
+      readBase64VLQ(*sourceMap) + 1; // adjust zero-based line number
+    uint32_t columnNumber = readBase64VLQ(*sourceMap);
+    nextDebugLocation = {fileIndex, lineNumber, columnNumber};
+    nextDebugLocationHasDebugInfo = true;
+  }
 }
 
 void WasmBinaryReader::readNextDebugLocation() {

test/gtest/CMakeLists.txt

@@ -2,6 +2,7 @@ include_directories(../../third_party/googletest/googletest/include)
 include_directories(../../src/wasm)
 
 set(unittest_SOURCES
+  binary-reader.cpp
   cfg.cpp
   dfa_minimization.cpp
   disjoint_sets.cpp

test/gtest/binary-reader.cpp

@@ -0,0 +1,63 @@
+/*
+ * Copyright 2024 WebAssembly Community Group participants
+ *
+ * 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.
+ */
+
+#include "parser/wat-parser.h"
+#include "print-test.h"
+#include "wasm-binary.h"
+#include "gtest/gtest.h"
+
+using namespace wasm;
+
+using BinaryReaderTest = PrintTest;
+
+// Check that debug location parsers can handle single-segment mappings.
+TEST_F(BinaryReaderTest, SourceMappingSingleSegment) {
+  auto moduleText = "(module)";
+  Module module;
+  parseWast(module, moduleText);
+
+  BufferWithRandomAccess buffer;
+  WasmBinaryWriter(&module, buffer, PassOptions());
+  auto moduleBytes = buffer.getAsChars();
+
+  // A single-segment mapping starting at offset 0.
+  std::string sourceMap = R"(
+      {
+          "version": 3,
+          "sources": [],
+          "names": [],
+          "mappings": "A"
+      }
+  )";
+  std::stringstream sourceMapStream(sourceMap);
+
+  // Test `readSourceMapHeader` (only check for errors, as there is no mapping
+  // to print).
+  {
+    Module module;
+    WasmBinaryReader binaryReader(module, FeatureSet::All, moduleBytes);
+    binaryReader.setDebugLocations(&sourceMapStream);
+    binaryReader.readSourceMapHeader();
+  }
+
+  // Test `readNextDebugLocation`.
+  {
+    Module module;
+    WasmBinaryReader binaryReader(module, FeatureSet::All, moduleBytes);
+    binaryReader.setDebugLocations(&sourceMapStream);
+    binaryReader.readNextDebugLocation();
+  }
+}