Update Windows trampolines for compatibility with Python 3.7 and earlier

crates/uv-install-wheel/src/wheel.rs

@@ -227,14 +227,14 @@ pub(crate) fn windows_script_launcher(
 
     let mut launcher: Vec<u8> = Vec::with_capacity(launcher_bin.len() + payload.len());
     launcher.extend_from_slice(launcher_bin);
-    launcher.extend_from_slice(&payload);
     launcher.extend_from_slice(python_path.as_bytes());
     launcher.extend_from_slice(
         &u32::try_from(python_path.as_bytes().len())
             .expect("File Path to be smaller than 4GB")
             .to_le_bytes(),
     );
     launcher.extend_from_slice(&LAUNCHER_MAGIC_NUMBER);
+    launcher.extend_from_slice(&payload);
 
     Ok(launcher)
 }

crates/uv-trampoline/README.md

@@ -88,19 +88,20 @@ Basically, this looks up `python.exe` (for console programs) and invokes
 
 The intended use is:
 
-- take your Python script, name it `__main__.py`, and pack it into a `.zip` file. Then concatenate
-  that `.zip` file onto the end of one of our prebuilt `.exe`s.
-- After the zip file content, write the path to the Python executable that the script uses to run
+- First, place our prebuilt `.exe` content at the top of the file.
+- After the exe file content, write the path to the Python executable that the script uses to run
   the Python script as UTF-8 encoded string, followed by the path's length as a 32-bit little-endian
   integer.
-- At the very end, write the magic number `UVUV` in bytes.
+- Write the magic number `UVUV` in bytes.
+- Finally, rename your Python script as `__main__.py`, compress it into a `.zip` file, and append
+  this `.zip` file to the end of one of our prebuilt `.exe` files.
 
 |       `launcher.exe`        |
 | :-------------------------: |
-|  `<zipped python script>`   |
 |   `<path to python.exe>`    |
 | `<len(path to python.exe)>` |
 | `<b'U', b'V', b'U', b'V'>`  |
+|  `<zipped python script>`   |
 
 Then when you run `python` on the `.exe`, it will see the `.zip` trailer at the end of the `.exe`,
 and automagically look inside to find and execute `__main__.py`. Easy-peasy.

crates/uv-trampoline/src/bounce.rs

@@ -87,11 +87,12 @@ fn push_quoted_path(path: &Path, command: &mut Vec<u8>) {
 }
 
 /// Reads the executable binary from the back to find the path to the Python executable that is written
-/// after the ZIP file content.
+/// before the ZIP file content.
 ///
 /// The executable is expected to have the following format:
-/// * The file must end with the magic number 'UVUV'.
-/// * The last 4 bytes (little endian) are the length of the path to the Python executable.
+/// * The last part of the file is ZIP file content.
+/// * The remain part must end with the magic number 'UVUV'.
+/// * The last 4 bytes (little endian) before 'UVUV' are the length of the path to the Python executable.
 /// * The path encoded as UTF-8 comes right before the length
 ///
 /// # Panics
@@ -112,6 +113,45 @@ fn find_python_exe(executable_name: &Path) -> PathBuf {
     });
     let file_size = metadata.len();
 
+    // Read the entire end of central directory (EOCD) of the ZIP file, which is 22 bytes long.
+    let mut eocd_buf: Vec<u8> = vec![0; 22];
+
+    file_handle
+        .seek(SeekFrom::Start(file_size - 22))
+        .unwrap_or_else(|_| {
+            print_last_error_and_exit("Failed to set the file pointer to the start of zip EOCD");
+        });
+
+    let read_bytes = file_handle.read(&mut eocd_buf).unwrap_or_else(|_| {
+        print_last_error_and_exit("Failed to read the zip EOCD");
+    });
+
+    if read_bytes != 22 {
+        eprintln!(
+            "Failed to read the EOCD. Expected 22 bytes but read {}",
+            read_bytes
+        );
+        exit_with_status(1);
+    }
+
+    // Size of the central directory (in bytes)
+    let cd_size = u32::from_le_bytes(eocd_buf[12..16].try_into().unwrap_or_else(|_| {
+        eprintln!("Slice length is not equal to 4 bytes");
+        exit_with_status(1);
+    }));
+
+    // Offset of central directory (unit is bytes).
+    // In other words, the number of bytes in the ZIP file at which the central directory starts.
+    let cd_offset = u32::from_le_bytes(eocd_buf[16..20].try_into().unwrap_or_else(|_| {
+        eprintln!("Slice length is not equal to 4 bytes");
+        exit_with_status(1);
+    }));
+
+    // Calculate the position in the entire executable where the content of the ZIP file begins.
+    let end_of_cd = file_size - 22;
+    let start_of_cd = end_of_cd - cd_size as u64;
+    let start_of_zip = start_of_cd - cd_offset as u64;
+
     // Start with a size of 1024 bytes which should be enough for most paths but avoids reading the
     // entire file.
     let mut buffer: Vec<u8> = Vec::new();
@@ -122,7 +162,7 @@ fn find_python_exe(executable_name: &Path) -> PathBuf {
         buffer.resize(bytes_to_read as usize, 0);
 
         file_handle
-            .seek(SeekFrom::Start(file_size - u64::from(bytes_to_read)))
+            .seek(SeekFrom::Start(start_of_zip - u64::from(bytes_to_read)))
             .unwrap_or_else(|_| {
                 print_last_error_and_exit("Failed to set the file pointer to the end of the file");
             });

crates/uv-trampoline/tests/harness.rs

@@ -175,14 +175,14 @@ fn windows_script_launcher(
 
     let mut launcher: Vec<u8> = Vec::with_capacity(launcher_bin.len() + payload.len());
     launcher.extend_from_slice(launcher_bin);
-    launcher.extend_from_slice(&payload);
     launcher.extend_from_slice(python_path.as_bytes());
     launcher.extend_from_slice(
         &u32::try_from(python_path.as_bytes().len())
             .expect("File Path to be smaller than 4GB")
             .to_le_bytes(),
     );
     launcher.extend_from_slice(&LAUNCHER_MAGIC_NUMBER);
+    launcher.extend_from_slice(&payload);
 
     Ok(launcher)
 }

crates/uv/tests/it/tool_install.rs

@@ -3112,3 +3112,60 @@ fn tool_install_at_latest_upgrade() {
         "###);
     });
 }
+
+#[cfg(windows)]
+#[test]
+fn tool_install_windows_3_7() {
+    let context = TestContext::new("3.7")
+        .with_filtered_counts()
+        .with_filtered_exe_suffix();
+    let tool_dir = context.temp_dir.child("tools");
+    let bin_dir = context.temp_dir.child("bin");
+
+    // Install `black`
+    uv_snapshot!(context.filters(), context.tool_install()
+        .arg("black")
+        .env(EnvVars::UV_TOOL_DIR, tool_dir.as_os_str())
+        .env(EnvVars::XDG_BIN_HOME, bin_dir.as_os_str())
+        .env(EnvVars::PATH, bin_dir.as_os_str()), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Resolved [N] packages in [TIME]
+    Prepared [N] packages in [TIME]
+    Installed [N] packages in [TIME]
+     + black==23.3.0
+     + click==8.1.7
+     + importlib-metadata==6.7.0
+     + mypy-extensions==1.0.0
+     + packaging==24.0
+     + pathspec==0.11.2
+     + platformdirs==4.0.0
+     + tomli==2.0.1
+     + typed-ast==1.5.5
+     + typing-extensions==4.7.1
+     + zipp==3.15.0
+    Installed 2 executables: black, blackd
+    "###);
+
+    tool_dir.child("black").assert(predicate::path::is_dir());
+    tool_dir
+        .child("black")
+        .child("uv-receipt.toml")
+        .assert(predicate::path::exists());
+
+    let executable = bin_dir.child(format!("black{}", std::env::consts::EXE_SUFFIX));
+    assert!(executable.exists());
+
+    uv_snapshot!(context.filters(), Command::new("black").arg("--version").env(EnvVars::PATH, bin_dir.as_os_str()), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    black, 23.3.0 (compiled: yes)
+    Python (CPython) 3.7.[X]
+
+    ----- stderr -----
+    "###);
+}