Preserve original PDB path in dll

Mono.Cecil.Cil/PortablePdb.cs

@@ -319,6 +319,20 @@ public void Write ()
 			}
 		}
 
+		string GetPdbPath ()
+		{
+			var debugHeader = module.Image.DebugHeader;
+			foreach (var entry in debugHeader.Entries) {
+				var data = entry.Data;
+				// Pdb path is NUL-terminated path at offset 24.
+				// https://github.com/dotnet/runtime/blob/main/docs/design/specs/PE-COFF.md#codeview-debug-directory-entry-type-2
+				if (entry.Directory.Type == ImageDebugType.CodeView && data.Length >= 25)
+					return System.Text.Encoding.UTF8.GetString (data, 24, data.Length - 25);
+			}
+
+			return string.Empty;
+		}
+
 		public ImageDebugHeader GetDebugHeader ()
 		{
 			if (IsEmbedded)
@@ -341,7 +355,10 @@ public ImageDebugHeader GetDebugHeader ()
 				// PDB Age
 				buffer.WriteUInt32 (1);
 				// PDB Path
-				var fileName = writer.BaseStream.GetFileName ();
+				var fileName = GetPdbPath ();
+				if (string.IsNullOrEmpty (fileName)) {
+					fileName = writer.BaseStream.GetFileName ();
+				}
 				if (string.IsNullOrEmpty (fileName)) {
 					fileName = module.Assembly.Name.Name + ".pdb";
 				}

Test/Mono.Cecil.Tests/PortablePdbTests.cs

@@ -1148,5 +1148,33 @@ static void GetCodeViewPdbId (ModuleDefinition module, out byte[] pdbId)
 			buffer.WriteInt32 (cv.Directory.TimeDateStamp);
 			pdbId = buffer.buffer;
 		}
+
+		[Test]
+		public void WritePortablePdbPath ()
+		{
+			const string resource = "PdbPathLib.dll";
+			string destination = Path.GetTempFileName ();
+
+			using (var module = GetResourceModule (resource, new ReaderParameters { ReadSymbols = true })) {
+				module.Write (destination, new WriterParameters { WriteSymbols = true });
+			}
+
+			using (var module = ModuleDefinition.ReadModule (destination, new ReaderParameters { ReadSymbols = true })) {
+				GetCodeViewPdbPath (module, out string pdbPath);
+
+				Assert.AreEqual ("/_/artifacts/obj/PdbPathLib/release/PdbPathLib.pdb", pdbPath);
+			}
+		}
+
+		static void GetCodeViewPdbPath (ModuleDefinition module, out string pdbPath)
+		{
+			var header = module.GetDebugHeader ();
+			var cv = Mixin.GetCodeViewEntry (header);
+			Assert.IsNotNull (cv);
+
+			CollectionAssert.AreEqual (new byte [] { 0x52, 0x53, 0x44, 0x53 }, cv.Data.Take (4));
+
+			pdbPath = Encoding.UTF8.GetString (cv.Data, 24, cv.Data.Length - 25);
+		}
 	}
 }