Make dbghelp look for PDBs next to their exe/dll.

.github/workflows/main.yml

@@ -110,6 +110,16 @@ jobs:
     - run: ./ci/debuglink-docker.sh
       if: contains(matrix.os, 'ubuntu')
 
+    # Test that backtraces are still symbolicated if we don't embed an absolute
+    # path to the PDB file in the binary.
+    - run: |
+        cargo clean
+        cargo test
+      if: contains(matrix.rust, 'msvc')
+      name: "Test that backtraces are symbolicated without absolute PDB path"
+      env:
+        RUSTFLAGS: "-C link-arg=/PDBALTPATH:%_PDB%"
+
     # Test that including as a submodule will still work, both with and without
     # the `backtrace` feature enabled.
     - run: cargo build --manifest-path crates/as-if-std/Cargo.toml

src/dbghelp.rs

@@ -23,9 +23,12 @@
 
 #![allow(non_snake_case)]
 
+use alloc::vec::Vec;
+
 use super::windows::*;
 use core::mem;
 use core::ptr;
+use core::slice;
 
 // Work around `SymGetOptions` and `SymSetOptions` not being present in winapi
 // itself. Otherwise this is only used when we're double-checking types against
@@ -65,6 +68,17 @@ mod dbghelp {
             CurContext: LPDWORD,
             CurFrameIndex: LPDWORD,
         ) -> BOOL;
+        pub fn SymGetSearchPathW(
+            hprocess: HANDLE,
+            searchpatha: PWSTR,
+            searchpathlength: u32,
+        ) -> BOOL;
+        pub fn SymSetSearchPathW(hprocess: HANDLE, searchpatha: PCWSTR) -> BOOL;
+        pub fn EnumerateLoadedModulesW64(
+            hprocess: HANDLE,
+            enumloadedmodulescallback: PENUMLOADED_MODULES_CALLBACKW64,
+            usercontext: *const c_void,
+        ) -> BOOL;
     }
 
     pub fn assert_equal_types<T>(a: T, _b: T) -> T {
@@ -174,6 +188,20 @@ dbghelp! {
             path: PCWSTR,
             invade: BOOL
         ) -> BOOL;
+        fn SymGetSearchPathW(
+            hprocess: HANDLE,
+            searchpatha: PWSTR,
+            searchpathlength: u32
+        ) -> BOOL;
+        fn SymSetSearchPathW(
+            hprocess: HANDLE,
+            searchpatha: PCWSTR
+        ) -> BOOL;
+        fn EnumerateLoadedModulesW64(
+            hprocess: HANDLE,
+            enumloadedmodulescallback: PENUMLOADED_MODULES_CALLBACKW64,
+            usercontext: *const c_void
+        ) -> BOOL;
         fn StackWalk64(
             MachineType: DWORD,
             hProcess: HANDLE,
@@ -372,11 +400,133 @@ pub fn init() -> Result<Init, ()> {
         // get to initialization first and the other will pick up that
         // initialization.
         DBGHELP.SymInitializeW().unwrap()(GetCurrentProcess(), ptr::null_mut(), TRUE);
+
+        // The default search path for dbghelp will only look in the current working
+        // directory and (possibly) `_NT_SYMBOL_PATH` and `_NT_ALT_SYMBOL_PATH`.
+        // However, we also want to look in the directory of the executable
+        // and each DLL that is loaded. To do this, we need to update the search path
+        // to include these directories.
+        //
+        // See https://learn.microsoft.com/cpp/build/reference/pdbpath for an
+        // example of where symbols are usually searched for.
+        let mut search_path_buf = Vec::new();
+        search_path_buf.resize(1024, 0);
+
+        // Prefill the buffer with the current search path.
+        if DBGHELP.SymGetSearchPathW().unwrap()(
+            GetCurrentProcess(),
+            search_path_buf.as_mut_ptr(),
+            search_path_buf.len() as _,
+        ) == TRUE
+        {
+            // Trim the buffer to the actual length of the string.
+            let len = lstrlenW(search_path_buf.as_mut_ptr());
+            assert!(len >= 0);
+            search_path_buf.truncate(len as usize);
+        } else {
+            // If getting the search path fails, at least include the current directory.
+            search_path_buf.clear();
+            search_path_buf.push(utf16_char('.'));
+            search_path_buf.push(utf16_char(';'));
+        }
+
+        let mut search_path = SearchPath::new(search_path_buf);
+
+        // Update the search path to include the directory of the executable and each DLL.
+        DBGHELP.EnumerateLoadedModulesW64().unwrap()(
+            GetCurrentProcess(),
+            Some(enum_loaded_modules_callback),
+            &mut search_path as *mut _ as *mut _,
+        );
+
+        let new_search_path = search_path.finalize();
+
+        // Set the new search path.
+        DBGHELP.SymSetSearchPathW().unwrap()(GetCurrentProcess(), new_search_path.as_ptr());
+
         INITIALIZED = true;
         Ok(ret)
     }
 }
 
+struct SearchPath {
+    search_path_utf16: Vec<u16>,
+}
+
+fn utf16_char(c: char) -> u16 {
+    let buf = &mut [0u16; 2];
+    let buf = c.encode_utf16(buf);
+    assert!(buf.len() == 1);
+    buf[0]
+}
+
+impl SearchPath {
+    fn new(initial_search_path: Vec<u16>) -> Self {
+        Self {
+            search_path_utf16: initial_search_path,
+        }
+    }
+
+    /// Add a path to the search path if it is not already present.
+    fn add(&mut self, path: &[u16]) {
+        let sep = utf16_char(';');
+
+        // We could deduplicate in a case-insensitive way, but case-sensitivity
+        // can be configured by directory on Windows, so let's not do that.
+        // https://learn.microsoft.com/windows/wsl/case-sensitivity
+        if !self
+            .search_path_utf16
+            .split(|&c| c == sep)
+            .any(|p| p == path)
+        {
+            if self.search_path_utf16.last() != Some(&sep) {
+                self.search_path_utf16.push(sep);
+            }
+            self.search_path_utf16.extend_from_slice(path);
+        }
+    }
+
+    fn finalize(mut self) -> Vec<u16> {
+        // Add a null terminator.
+        self.search_path_utf16.push(0);
+        self.search_path_utf16
+    }
+}
+
+extern "system" fn enum_loaded_modules_callback(
+    module_name: PCWSTR,
+    _: u64,
+    _: u32,
+    user_context: *const c_void,
+) -> BOOL {
+    // `module_name` is an absolute path like `C:\path\to\module.dll`
+    // or `C:\path\to\module.exe`
+    let len: usize = unsafe { lstrlenW(module_name).try_into().unwrap() };
+
+    if len == 0 {
+        // This should not happen, but if it does, we can just ignore it.
+        return TRUE;
+    }
+
+    let module_name = unsafe { slice::from_raw_parts(module_name, len) };
+    let path_sep = utf16_char('\\');
+    let alt_path_sep = utf16_char('/');
+
+    let Some(end_of_directory) = module_name
+        .iter()
+        .rposition(|&c| c == path_sep || c == alt_path_sep)
+    else {
+        // `module_name` being an absolute path, it should always contain at least one
+        // path separator. If not, there is nothing we can do.
+        return TRUE;
+    };
+
+    let search_path = unsafe { &mut *(user_context as *mut SearchPath) };
+    search_path.add(&module_name[..end_of_directory]);
+
+    TRUE
+}
+
 impl Drop for Init {
     fn drop(&mut self) {
         unsafe {

src/windows.rs

@@ -54,6 +54,16 @@ cfg_if::cfg_if! {
                     ContextPointers: PKNONVOLATILE_CONTEXT_POINTERS
                 ) -> PEXCEPTION_ROUTINE;
             }
+
+            // winapi doesn't have this type
+            pub type PENUMLOADED_MODULES_CALLBACKW64 = Option<
+            unsafe extern "system" fn(
+                modulename: PCWSTR,
+                modulebase: u64,
+                modulesize: u32,
+                usercontext: *const c_void,
+            ) -> BOOL,
+        >;
         }
     } else {
         pub use core::ffi::c_void;
@@ -291,6 +301,7 @@ ffi! {
     pub type PTRANSLATE_ADDRESS_ROUTINE64 = Option<
         unsafe extern "system" fn(hProcess: HANDLE, hThread: HANDLE, lpaddr: LPADDRESS64) -> DWORD64,
     >;
+    pub type PENUMLOADED_MODULES_CALLBACKW64 = Option<unsafe extern "system" fn(modulename: PCWSTR, modulebase: u64, modulesize: u32, usercontext: *const c_void) -> BOOL>;
     pub type PGET_MODULE_BASE_ROUTINE64 =
         Option<unsafe extern "system" fn(hProcess: HANDLE, Address: DWORD64) -> DWORD64>;
     pub type PFUNCTION_TABLE_ACCESS_ROUTINE64 =
@@ -444,6 +455,7 @@ ffi! {
             hSnapshot: HANDLE,
             lpme: LPMODULEENTRY32W,
         ) -> BOOL;
+        pub fn lstrlenW(lpstring: PCWSTR) -> i32;
     }
 }