AOT publish passes with long paths on Windows

src/mono/mono/eglib/gfile-win32.c

@@ -93,12 +93,16 @@ gboolean
 g_file_test (const gchar *filename, GFileTest test)
 {
 	gunichar2* utf16_filename = NULL;
+	gchar *filename_with_prefix = NULL;
 	DWORD attr;
 
 	if (filename == NULL || test == 0)
 		return FALSE;
 
-	utf16_filename = u8to16 (filename);
+	filename_with_prefix = g_path_make_long_compatible (filename);
+	utf16_filename = u8to16 (filename_with_prefix);
+	g_free (filename_with_prefix);
+
 	attr = GetFileAttributesW (utf16_filename);
 	g_free (utf16_filename);
 

src/mono/mono/eglib/gfile.c

@@ -127,10 +127,13 @@ g_fopen (const gchar *path, const gchar *mode)
 		return NULL;
 
 #ifdef HOST_WIN32
-	if (is_ascii_string (path) && is_ascii_string (mode)) {
-		fp = fopen (path, mode);
+	gchar *path_mod;
+	path_mod = g_path_make_long_compatible(path);
+
+	if (is_ascii_string (path_mod) && is_ascii_string (mode)) {
+		fp = fopen (path_mod, mode);
 	} else {
-		gunichar2 *wPath = g_utf8_to_utf16 (path, -1, 0, 0, 0);
+		gunichar2 *wPath = g_utf8_to_utf16 (path_mod, -1, 0, 0, 0);
 		gunichar2 *wMode = g_utf8_to_utf16 (mode, -1, 0, 0, 0);
 
 		if (!wPath || !wMode)
@@ -140,6 +143,7 @@ g_fopen (const gchar *path, const gchar *mode)
 		g_free (wPath);
 		g_free (wMode);
 	}
+	g_free (path_mod);
 #else
 	fp = fopen (path, mode);
 #endif

src/mono/mono/eglib/glib.h

@@ -898,6 +898,10 @@ gchar  *g_path_get_basename    (const char *filename);
 gchar  *g_get_current_dir      (void);
 gboolean g_path_is_absolute    (const char *filename);
 
+#ifdef G_OS_WIN32
+gchar  *g_path_make_long_compatible (const gchar *path);
+#endif
+
 const gchar *g_get_tmp_dir     (void);
 
 gboolean g_ensure_directory_exists (const gchar *filename);

src/mono/mono/eglib/gpath.c

@@ -33,12 +33,114 @@
 
 #ifdef G_OS_WIN32
 #include <direct.h>
+#include <windows.h>
 #endif
 
 #ifdef HAVE_UNISTD_H
 #include <unistd.h>
 #endif
 
+#ifdef G_OS_WIN32
+
+#ifndef MAX_PATH
+#define MAX_PATH 260
+#endif
+
+/* Helper function to check if a Windows path needs the \\?\ prefix for long path support.
+ * Returns TRUE if:
+ * - The path is long enough to potentially hit MAX_PATH limit
+ * - The path doesn't already have the \\?\ prefix
+ * - The path is an absolute Windows path (e.g., C:\path), UNC path (e.g., \\server\share),
+ *   or drive-relative path (e.g., \Windows\System32)
+ */
+static gboolean
+g_path_needs_long_prefix (const gchar *path)
+{
+	if (!path || strlen(path) <= 2)
+		return FALSE;
+
+	/* Only add prefix for paths that are approaching or exceeding MAX_PATH */
+	if (strlen(path) < MAX_PATH)
+		return FALSE;
+
+	if (strncmp(path, "\\\\?\\", 4) == 0)
+		return FALSE;
+
+	if (path[1] == ':' && (path[2] == '\\' || path[2] == '/'))
+		return TRUE;
+
+	if (path[0] == '\\' && path[1] == '\\' && path[2] != '?')
+		return TRUE;
+
+	if (path[0] == '\\' && path[1] != '\\')
+		return TRUE;
+
+	return FALSE;
+}
+
+/* Helper function to convert a drive-relative path to an absolute path.
+ * For example, \Windows\System32 becomes C:\Windows\System32.
+ * Caller must free the result with g_free().
+ */
+static gchar *
+g_path_drive_relative_to_absolute (const gchar *path)
+{
+	if (!path)
+		return g_strdup(path);
+
+	char current_dir[MAX_PATH];
+	if (GetCurrentDirectoryA(MAX_PATH, current_dir) > 0 && current_dir[1] == ':') {
+		gchar *result = g_malloc(strlen(path) + 3);
+		result[0] = current_dir[0];
+		result[1] = ':';
+		strcpy(result + 2, path);
+		return result;
+	}
+
+	return g_strdup(path);
+}
+
+/* Makes a path compatible with long path support by adding \\?\ prefix if needed. Caller must free the result. */
+gchar *
+g_path_make_long_compatible (const gchar *path)
+{
+	if (!path)
+		return NULL;
+
+	gchar *work_path;
+
+	/* Drive-relative paths (e.g., \Windows\System32) need to be converted to absolute paths first */
+	if (path[0] == '\\' && path[1] != '\\') {
+		work_path = g_path_drive_relative_to_absolute(path);
+	} else {
+		work_path = g_strdup(path);
+	}
+
+	gchar *result;
+
+	if (!g_path_needs_long_prefix(work_path)) {
+		g_free(work_path);
+		return g_strdup(path);
+	}
+
+	/* Handle UNC paths: \\server\share becomes \\?\UNC\server\share */
+	if (work_path[0] == '\\' && work_path[1] == '\\') {
+		result = g_malloc(strlen(work_path) + 7);
+		strcpy(result, "\\\\?\\UNC\\");
+		strcat(result, work_path + 2);
+		g_free(work_path);
+		return result;
+	}
+
+	/* Handle absolute paths: C:\path becomes \\?\C:\path */
+	result = g_malloc(strlen(work_path) + 5);
+	strcpy(result, "\\\\?\\");
+	strcat(result, work_path);
+	g_free(work_path);
+	return result;
+}
+#endif
+
 gchar *
 g_build_path (const gchar *separator, const gchar *first_element, ...)
 {

src/mono/mono/metadata/image.c

@@ -1765,17 +1765,19 @@ mono_image_open_a_lot_parameterized (MonoLoadedImages *li, MonoAssemblyLoadConte
 	 */
 	mono_images_lock ();
 	image = (MonoImage *)g_hash_table_lookup (loaded_images, absfname);
-	g_free (absfname);
 
 	if (image) { // Image already loaded
 		mono_image_addref (image);
 		mono_images_unlock ();
+		g_free (absfname);
 		return image;
 	}
 	mono_images_unlock ();
 
 	// Image not loaded, load it now
-	image = do_mono_image_open (alc, fname, status, options);
+	// Use absfname (the resolved absolute path) instead of fname (which may be relative)
+	image = do_mono_image_open (alc, absfname, status, options);
+	g_free (absfname);
 	if (image == NULL)
 		return NULL;
 

src/mono/mono/utils/mono-path.c

@@ -99,6 +99,12 @@ mono_path_canonicalize (const char *path)
 		abspath [len+1] = 0;
 	}
 
+#ifdef HOST_WIN32
+	gchar *prefixed = g_path_make_long_compatible(abspath);
+	g_free(abspath);
+	abspath = prefixed;
+#endif
+
 	return abspath;
 }
 

src/mono/wasm/Wasm.Build.Tests/Blazor/BuildPublishTests.cs

@@ -10,6 +10,7 @@
 using Xunit.Sdk;
 using Microsoft.Playwright;
 using System.Runtime.InteropServices;
+using System;
 
 #nullable enable
 
@@ -33,8 +34,6 @@ public static TheoryData<Configuration, bool> TestDataForDefaultTemplate_WithWor
             data.Add(Configuration.Debug, true);
         }
 
-        // [ActiveIssue("https://github.com/dotnet/runtime/issues/103625", TestPlatforms.Windows)]
-        // when running locally the path might be longer than 260 chars and these tests can fail with AOT
         data.Add(Configuration.Release, false); // Release relinks by default
         data.Add(Configuration.Release, true);
         return data;
@@ -59,6 +58,51 @@ public void DefaultTemplate_AOT_WithWorkload(Configuration config, bool testUnic
         PublishProject(info, config, new PublishOptions(AOT: true, UseCache: false));
     }
 
+    [Fact]
+    public void DefaultTemplate_AOT_WithLongPath()
+    {
+        Configuration config = Configuration.Release;
+        ProjectInfo info = CopyTestAsset(config, aot: false, TestAsset.BlazorBasicTestApp, "lp", appendUnicodeToPath: true);
+
+        // Move the test project into a nested directory to create a long path that exceeds MAX_PATH
+        info = NestProjectInLongPath(info);
+
+        BlazorBuild(info, config);
+        PublishProject(info, config, new PublishOptions(AOT: true, UseCache: false));
+    }
+
+    private ProjectInfo NestProjectInLongPath(ProjectInfo info)
+    {
+        string testProjectDir = Path.GetDirectoryName(_projectDir)!;
+        string testProjectDirName = Path.GetFileName(testProjectDir);
+        string baseDir = Path.GetDirectoryName(testProjectDir)!;
+
+        // The problematic path is in a form of:
+        // {_projectDir}\obj\..\Microsoft_Extensions_DependencyInjection_dll_compiled_methods.txt
+        // Its length is at least 100 characters, so we can subtract it from the nesting target
+        const int longestBuildSubpathLength = 100;
+        const int windowsMaxPath = 260;
+
+        int currentLength = _projectDir.Length;
+        int targetPathLength = windowsMaxPath - longestBuildSubpathLength;
+        int additionalLength = Math.Max(0, targetPathLength - currentLength);
+
+        if (additionalLength == 0)
+            return info;
+
+        string dirName = new string('x', additionalLength);
+        string nestedPath = Path.Combine(baseDir, dirName);
+        Directory.CreateDirectory(nestedPath);
+
+        string newTestProjectDir = Path.Combine(nestedPath, testProjectDirName);
+        Directory.Move(testProjectDir, newTestProjectDir);
+
+        _projectDir = Path.Combine(newTestProjectDir, "App");
+        string nestedProjectFilePath = Path.Combine(_projectDir, "BlazorBasicTestApp.csproj");
+
+        return new ProjectInfo(info.ProjectName, nestedProjectFilePath, info.LogPath, info.NugetDir);
+    }
+
     // Disabling for now - publish folder can have more than one dotnet*hash*js, and not sure
     // how to pick which one to check, for the test
     //[Theory]

src/tasks/AotCompilerTask/MonoAOTCompiler.cs

@@ -1004,16 +1004,21 @@ private PrecompileArguments GetPrecompileArgumentsFor(ITaskItem assemblyItem, st
         }
         else
         {
+            string assemblyPath;
             if (string.IsNullOrEmpty(WorkingDirectory))
             {
-                processArgs.Add('"' + assemblyFilename + '"');
+                // Pass the full assembly path to the AOT compiler to support long paths (> MAX_PATH)
+                assemblyPath = assembly;
             }
             else
             {
                 // If WorkingDirectory is supplied, the caller could be passing in a relative path
-                // Use the original ItemSpec that was passed in.
-                processArgs.Add('"' + assemblyItem.ItemSpec + '"');
+                // Convert to absolute path to ensure long path support works correctly
+                assemblyPath = Path.IsPathFullyQualified(assemblyItem.ItemSpec)
+                    ? assemblyItem.ItemSpec
+                    : Path.GetFullPath(Path.Combine(WorkingDirectory, assemblyItem.ItemSpec));
             }
+            processArgs.Add('"' + assemblyPath + '"');
         }
 
         monoPaths = $"{assemblyDir}{Path.PathSeparator}{monoPaths}";