[LLDB] Improve PDB discovery

When loading a PE/COFF target, the associated PDB file often wasn't
found.  The executable module contains a path for the associated PDB
file, but people often debug from a different directory than the one
their build system uses.  (This is especially common in post-mortem
and cross platform debugging.)

Suppose the COFF executable being debugged is `~/proj/foo.exe`, but
it was built elsewhere and refers to `D:\remote\build\env\foobar.pdb`,
LLDB wouldn't find it.

With this change, if no file exists at the PDB path, LLDB will look
in the executable directory for a PDB file that matches the name of
the one it expected (e.g., `~/proj/foobar.pdb`).  If found, the PDB
is subject to the same matching criteria (GUIDs and age) as would
have been used had it been in the original location.

This same-directory-as-the-binary rule is commonly used by debuggers
on Windows.

Differential Review: https://reviews.llvm.org/D84815

Author: amccarth-google

lldb/source/Plugins/SymbolFile/NativePDB/SymbolFileNativePDB.cpp

@@ -134,8 +134,16 @@ loadMatchingPDBFile(std::string exe_path, llvm::BumpPtrAllocator &allocator) {
     return nullptr;
   }
 
-  // if the file doesn't exist, is not a pdb, or doesn't have a matching guid,
-  // fail.
+  // If the file doesn't exist, perhaps the path specified at build time
+  // doesn't match the PDB's current location, so check the location of the
+  // executable.
+  if (!FileSystem::Instance().Exists(pdb_file)) {
+    const auto exe_dir = FileSpec(exe_path).CopyByRemovingLastPathComponent();
+    const auto pdb_name = FileSpec(pdb_file).GetFilename().GetCString();
+    pdb_file = exe_dir.CopyByAppendingPathComponent(pdb_name).GetCString();
+  }
+
+  // If the file is not a PDB or if it doesn't have a matching GUID, fail.
   llvm::file_magic magic;
   auto ec = llvm::identify_magic(pdb_file, magic);
   if (ec || magic != llvm::file_magic::pdb)

lldb/test/Shell/SymbolFile/NativePDB/Inputs/locate-pdb.lldbinit

@@ -0,0 +1,2 @@
+target modules dump symfile
+quit

lldb/test/Shell/SymbolFile/NativePDB/locate-pdb.cpp

@@ -0,0 +1,34 @@
+// clang-format off
+// REQUIRES: lld, x86
+
+// Test that lldb can find the PDB file that corresponds to the executable.  The linker
+// writes a path to the PDB in the executable.  If the PDB is not there, lldb should
+// check the directory that contains the executable.  We'll generate the PDB file in
+// a subdirectory and then move it into the directory with the executable.  That will
+// ensure the PDB path stored in the executable is wrong.
+
+// Build an EXE and PDB in different directories
+// RUN: mkdir -p %t/executable
+// RUN: rm -f %t/executable/foo.exe %t/executable/bar.pdb
+// RUN: mkdir -p %t/symbols
+// RUN: rm -f %t/symbols/bar.pdb
+// RUN: %clang_cl --target=x86_64-windows-msvc -Od -Z7 -c /Fo%t/executable/foo.obj -- %s
+// RUN: lld-link -debug:full -nodefaultlib -entry:main %t/executable/foo.obj \
+// RUN:     -out:%t/executable/foo.exe -pdb:%t/symbols/bar.pdb
+
+// Find the PDB in its build location
+// RUN: env LLDB_USE_NATIVE_PDB_READER=1 %lldb -f %t/executable/foo.exe -s \
+// RUN:     %p/Inputs/locate-pdb.lldbinit | FileCheck %s
+
+// Also find the PDB when it's adjacent to the executable
+// RUN: mv -f %t/symbols/bar.pdb %t/executable/bar.pdb
+// RUN: env LLDB_USE_NATIVE_PDB_READER=1 %lldb -f %t/executable/foo.exe -s \
+// RUN:     %p/Inputs/locate-pdb.lldbinit | FileCheck %s
+
+int main(int argc, char** argv) {
+  return 0;
+}
+
+// CHECK: (lldb) target modules dump symfile
+// CHECK: Dumping debug symbols for 1 modules.
+// CHECK: SymbolFile pdb