Always spawn a main2 thread to normalize main stack size issues (#10479)

Also removes UV_STACK_SIZE and uses RUST_MIN_STACK instead, tweaking
docs to reflect the differences.

Fixes #10367

.github/workflows/ci.yml

@@ -305,9 +305,6 @@ jobs:
 
       - name: "Smoke test"
         working-directory: ${{ env.UV_WORKSPACE }}
-        env:
-          # Avoid debug build stack overflows.
-          UV_STACK_SIZE: 3000000 # 3 megabyte, triple the default on windows
         run: |
           Set-Alias -Name uv -Value ./target/debug/uv
           uv venv -v
@@ -316,9 +313,6 @@ jobs:
       - name: "Smoke test completion"
         working-directory: ${{ env.UV_WORKSPACE }}
         shell: powershell
-        env:
-          # Avoid debug build stack overflows.
-          UV_STACK_SIZE: 3000000 # 3 megabyte, triple the default on windows
         run: |
           Set-Alias -Name uv -Value ./target/debug/uv
           Set-Alias -Name uvx -Value ./target/debug/uvx
@@ -744,9 +738,6 @@ jobs:
     needs: build-binary-windows
     name: "integration test | free-threaded on windows"
     runs-on: windows-latest
-    env:
-      # Avoid debug build stack overflows.
-      UV_STACK_SIZE: 3000000 # 3 megabyte, triple the default on windows
 
     steps:
       - name: "Download binary"
@@ -908,9 +899,6 @@ jobs:
           & .venv\Scripts\python.exe --version
 
       - name: "Check install"
-        env:
-          # Avoid debug build stack overflows.
-          UV_STACK_SIZE: 3000000 # 3 megabyte, triple the default on windows
         run: |
           .\uv.exe pip install anyio
 
@@ -1040,9 +1028,6 @@ jobs:
           & .venv\Scripts\python.exe --version
 
       - name: "Check install"
-        env:
-          # Avoid debug build stack overflows.
-          UV_STACK_SIZE: 3000000 # 3 megabyte, triple the default on windows
         run: |
           .\uv.exe pip install anyio
 
@@ -1534,9 +1519,6 @@ jobs:
     needs: build-binary-windows
     name: "check system | python3.10 on windows"
     runs-on: windows-latest
-    env:
-      # Avoid debug build stack overflows.
-      UV_STACK_SIZE: 3000000 # 3 megabyte, triple the default on windows
     steps:
       - uses: actions/checkout@v4
 
@@ -1560,9 +1542,6 @@ jobs:
     needs: build-binary-windows
     name: "check system | python3.10 on windows x86"
     runs-on: windows-latest
-    env:
-      # Avoid debug build stack overflows.
-      UV_STACK_SIZE: 3000000 # 3 megabyte, triple the default on windows
     steps:
       - uses: actions/checkout@v4
 
@@ -1587,9 +1566,6 @@ jobs:
     needs: build-binary-windows
     name: "check system | python3.13 on windows"
     runs-on: windows-latest
-    env:
-      # Avoid debug build stack overflows.
-      UV_STACK_SIZE: 3000000 # 3 megabyte, triple the default on windows
     steps:
       - uses: actions/checkout@v4
 
@@ -1615,9 +1591,6 @@ jobs:
     needs: build-binary-windows
     name: "check system | python3.12 via chocolatey"
     runs-on: windows-latest
-    env:
-      # Avoid debug build stack overflows.
-      UV_STACK_SIZE: 3000000 # 3 megabyte, triple the default on windows
     steps:
       - uses: actions/checkout@v4
 
@@ -1750,9 +1723,6 @@ jobs:
 
       - name: "Validate global Python install"
         shell: bash -el {0}
-        env:
-          # Avoid debug build stack overflows.
-          UV_STACK_SIZE: 3000000 # 3 megabyte, triple the default on windows
         run: python ./scripts/check_system_python.py --uv ./uv
 
   system-test-amazonlinux:
@@ -1789,9 +1759,6 @@ jobs:
     needs: build-binary-windows
     name: "check system | embedded python3.10 on windows"
     runs-on: windows-latest
-    env:
-      # Avoid debug build stack overflows.
-      UV_STACK_SIZE: 3000000 # 3 megabyte, triple the default on windows
     steps:
       - uses: actions/checkout@v4
 

CONTRIBUTING.md

@@ -82,18 +82,6 @@ cargo run -- venv
 cargo run -- pip install requests
 ```
 
-### Testing on Windows
-
-When testing debug builds on Windows, the stack can overflow resulting in a `STATUS_STACK_OVERFLOW`
-error code. This is due to a small stack size limit on Windows that we encounter when running
-unoptimized builds — the release builds do not have this problem. We
-[added a `UV_STACK_SIZE` variable](https://github.com/astral-sh/uv/pull/941) to bypass this problem
-during testing. We recommend bumping the stack size from the default of 1MB to 3MB, for example:
-
-```powershell
-$Env:UV_STACK_SIZE = '3000000'
-```
-
 ## Running inside a Docker container
 
 Source distributions can run arbitrary code on build and can make unwanted modifications to your

crates/uv-static/src/env_vars.rs

@@ -249,9 +249,6 @@ impl EnvVars {
     /// Use to disable line wrapping for diagnostics.
     pub const UV_NO_WRAP: &'static str = "UV_NO_WRAP";
 
-    /// Use to increase the stack size used by uv in debug builds on Windows.
-    pub const UV_STACK_SIZE: &'static str = "UV_STACK_SIZE";
-
     /// Generates the environment variable key for the HTTP Basic authentication username.
     #[attr_env_var_pattern("UV_INDEX_{name}_USERNAME")]
     pub fn index_username(name: &str) -> String {
@@ -505,6 +502,15 @@ impl EnvVars {
     /// for more.
     pub const RUST_LOG: &'static str = "RUST_LOG";
 
+    /// Use to set the stack size used by uv.
+    ///
+    /// The value is in bytes, and the default is typically 2MB (2097152).
+    /// Unlike the normal `RUST_MIN_STACK` semantics, this can affect main thread
+    /// stack size, because we actually spawn our own main2 thread to work around
+    /// the fact that Windows' real main thread is only 1MB. That thread has size
+    /// `max(RUST_MIN_STACK, 4MB)`.
+    pub const RUST_MIN_STACK: &'static str = "RUST_MIN_STACK";
+
     /// The directory containing the `Cargo.toml` manifest for a package.
     #[attr_hidden]
     pub const CARGO_MANIFEST_DIR: &'static str = "CARGO_MANIFEST_DIR";

crates/uv/src/lib.rs

@@ -1819,45 +1819,51 @@ where
         }
     };
 
-    // Windows has a default stack size of 1MB, which is lower than the linux and mac default.
+    // Running out of stack has been an issue for us. We box types and futures in various places
+    // to mitigate this, with this being an especially important case.
+    //
+    // Non-main threads should all have 2MB, as Rust forces platform consistency there,
+    // but that can be overridden with the RUST_MIN_STACK environment variable if you need more.
+    //
+    // Main thread stack-size is the real issue. There's BIG variety here across platforms
+    // and it's harder to control (which is why Rust doesn't by default). Notably
+    // on macOS and Linux you will typically get 8MB main thread, while on Windows you will
+    // typically get 1MB, which is *tiny*:
     // https://learn.microsoft.com/en-us/cpp/build/reference/stack-stack-allocations?view=msvc-170
-    // We support increasing the stack size to avoid stack overflows in debug mode on Windows. In
-    // addition, we box types and futures in various places. This includes the `Box::pin(run())`
-    // here, which prevents the large (non-send) main future alone from overflowing the stack.
-    let result = if let Ok(stack_size) = std::env::var(EnvVars::UV_STACK_SIZE) {
-        let stack_size = stack_size.parse().expect("Invalid stack size");
-        let tokio_main = move || {
-            let runtime = tokio::runtime::Builder::new_current_thread()
-                .enable_all()
-                .thread_stack_size(stack_size)
-                .build()
-                .expect("Failed building the Runtime");
-            // Box the large main future to avoid stack overflows.
-            let result = runtime.block_on(Box::pin(run(cli)));
-            // Avoid waiting for pending tasks to complete.
-            //
-            // The resolver may have kicked off HTTP requests during resolution that
-            // turned out to be unnecessary. Waiting for those to complete can cause
-            // the CLI to hang before exiting.
-            runtime.shutdown_background();
-            result
-        };
-        std::thread::Builder::new()
-            .stack_size(stack_size)
-            .spawn(tokio_main)
-            .expect("Tokio executor failed, was there a panic?")
-            .join()
-            .expect("Tokio executor failed, was there a panic?")
-    } else {
+    //
+    // To normalize this we just spawn a new thread called main2 with a size we can set
+    // ourselves. 2MB is typically too small (especially for our debug builds), while 4MB
+    // seems fine. Also we still try to respect RUST_MIN_STACK if it's set, in case useful,
+    // but don't let it ask for a smaller stack to avoid messy misconfiguration since we
+    // know we use quite a bit of main stack space.
+    let main_stack_size = std::env::var(EnvVars::RUST_MIN_STACK)
+        .ok()
+        .and_then(|var| var.parse::<usize>().ok())
+        .unwrap_or(0)
+        .max(4 * 1024 * 1024);
+
+    let main2 = move || {
         let runtime = tokio::runtime::Builder::new_current_thread()
             .enable_all()
             .build()
             .expect("Failed building the Runtime");
         // Box the large main future to avoid stack overflows.
         let result = runtime.block_on(Box::pin(run(cli)));
+        // Avoid waiting for pending tasks to complete.
+        //
+        // The resolver may have kicked off HTTP requests during resolution that
+        // turned out to be unnecessary. Waiting for those to complete can cause
+        // the CLI to hang before exiting.
         runtime.shutdown_background();
         result
     };
+    let result = std::thread::Builder::new()
+        .name("main2".to_owned())
+        .stack_size(main_stack_size)
+        .spawn(main2)
+        .expect("Tokio executor failed, was there a panic?")
+        .join()
+        .expect("Tokio executor failed, was there a panic?");
 
     match result {
         Ok(code) => code.into(),

crates/uv/tests/it/common/mod.rs

@@ -489,12 +489,6 @@ impl TestContext {
             // Avoid locale issues in tests
             command.env(EnvVars::LC_ALL, "C");
         }
-
-        if cfg!(all(windows, debug_assertions)) {
-            // TODO(konstin): Reduce stack usage in debug mode enough that the tests pass with the
-            // default windows stack of 1MB
-            command.env(EnvVars::UV_STACK_SIZE, (4 * 1024 * 1024).to_string());
-        }
     }
 
     /// Create a `pip compile` command for testing.
@@ -581,13 +575,6 @@ impl TestContext {
         let mut command = self.new_command();
         command.arg("help");
         command.env_remove(EnvVars::UV_CACHE_DIR);
-
-        if cfg!(all(windows, debug_assertions)) {
-            // TODO(konstin): Reduce stack usage in debug mode enough that the tests pass with the
-            // default windows stack of 1MB
-            command.env(EnvVars::UV_STACK_SIZE, (4 * 1024 * 1024).to_string());
-        }
-
         command
     }
 
@@ -636,13 +623,6 @@ impl TestContext {
     pub fn publish(&self) -> Command {
         let mut command = self.new_command();
         command.arg("publish");
-
-        if cfg!(all(windows, debug_assertions)) {
-            // TODO(konstin): Reduce stack usage in debug mode enough that the tests pass with the
-            // default windows stack of 1MB
-            command.env(EnvVars::UV_STACK_SIZE, (4 * 1024 * 1024).to_string());
-        }
-
         command
     }
 

crates/uv/tests/it/ecosystem.rs

@@ -104,11 +104,6 @@ fn lock_ecosystem_package(python_version: &str, name: &str) -> Result<()> {
         .env(EnvVars::UV_EXCLUDE_NEWER, EXCLUDE_NEWER)
         .current_dir(context.temp_dir.path());
 
-    if cfg!(all(windows, debug_assertions)) {
-        // TODO(konstin): Reduce stack usage in debug mode enough that the tests pass with the
-        // default windows stack of 1MB
-        command.env(EnvVars::UV_STACK_SIZE, (4 * 1024 * 1024).to_string());
-    }
     let (snapshot, _) = common::run_and_format(
         &mut command,
         context.filters(),

crates/uv/tests/it/show_settings.rs

@@ -24,13 +24,6 @@ fn add_shared_args(mut command: Command, cwd: &Path) -> Command {
         // Avoid locale issues in tests
         command.env(EnvVars::LC_ALL, "C");
     }
-
-    if cfg!(all(windows, debug_assertions)) {
-        // TODO(konstin): Reduce stack usage in debug mode enough that the tests pass with the
-        // default windows stack of 1MB
-        command.env(EnvVars::UV_STACK_SIZE, (4 * 1024 * 1024).to_string());
-    }
-
     command
 }
 

docs/configuration/environment.md

@@ -320,10 +320,6 @@ uv will require that all dependencies have a hash specified in the requirements
 Equivalent to the `--resolution` command-line argument. For example, if set to
 `lowest-direct`, uv will install the lowest compatible versions of all direct dependencies.
 
-### `UV_STACK_SIZE`
-
-Use to increase the stack size used by uv in debug builds on Windows.
-
 ### `UV_SYSTEM_PYTHON`
 
 Equivalent to the `--system` command-line argument. If set to `true`, uv will
@@ -484,6 +480,16 @@ For example:
 See the [tracing documentation](https://docs.rs/tracing-subscriber/latest/tracing_subscriber/filter/struct.EnvFilter.html#example-syntax)
 for more.
 
+### `RUST_MIN_STACK`
+
+Use to set the stack size used by uv.
+
+The value is in bytes, and the default is typically 2MB (2097152).
+Unlike the normal `RUST_MIN_STACK` semantics, this can affect main thread
+stack size, because we actually spawn our own main2 thread to work around
+the fact that Windows' real main thread is only 1MB. That thread has size
+`max(RUST_MIN_STACK, 4MB)`.
+
 ### `SHELL`
 
 The standard `SHELL` posix env var.