refactor(core): improve iOS log messages from stdout/stderr (#14385)

* refactor(core): improve iOS log messages from stdout/stderr

move the stdout/stderr forward logic to Swift so it does not consume a Rust thread and never deadlocks on the simulator

I had to work on this because i'm facing #12172 again on latest Xcode (26.1)

* patch

Author: lucasfernog

.changes/refactor-ios-stdout-stderr.md

@@ -0,0 +1,6 @@
+---
+"tauri": patch:bug
+"tauri-macros": patch:bug
+---
+
+Fix iOS deadlock when running on the simulator from Xcode by properly piping stdout/stderr messages through the Xcode console and OSLog.

crates/tauri/mobile/ios-api/Sources/Tauri/Logger.swift

@@ -4,6 +4,86 @@
 
 import os.log
 import UIKit
+import Foundation
+
+class StdoutRedirector {
+  private var originalStdout: Int32 = -1
+  private var originalStderr: Int32 = -1
+  private var stdoutPipe: [Int32] = [-1, -1]
+  private var stderrPipe: [Int32] = [-1, -1]
+  private var stdoutReadSource: DispatchSourceRead?
+  private var stderrReadSource: DispatchSourceRead?
+    
+  func start() {
+    originalStdout = dup(STDOUT_FILENO)
+    originalStderr = dup(STDERR_FILENO)
+        
+    guard Darwin.pipe(&stdoutPipe) == 0,
+      Darwin.pipe(&stderrPipe) == 0 else {
+      Logger.error("Failed to create stdout/stderr pipes")
+      return
+    }
+        
+    dup2(stdoutPipe[1], STDOUT_FILENO)
+    dup2(stderrPipe[1], STDERR_FILENO)
+    close(stdoutPipe[1])
+    close(stderrPipe[1])
+        
+    stdoutReadSource = createReader(
+      readPipe: stdoutPipe[0],
+      writeToOriginal: originalStdout,
+      label: "stdout"
+    )
+        
+    stderrReadSource = createReader(
+      readPipe: stderrPipe[0],
+      writeToOriginal: originalStderr,
+      label: "stderr"
+    )
+  }
+    
+  private func createReader(
+    readPipe: Int32,
+    writeToOriginal: Int32,
+    label: String
+  ) -> DispatchSourceRead {
+    let source = DispatchSource.makeReadSource(
+      fileDescriptor: readPipe,
+      queue: .global(qos: .utility)
+    )
+        
+    source.setEventHandler {
+      let bufferSize = 4096
+      var buffer = [UInt8](repeating: 0, count: bufferSize)
+      let bytesRead = read(readPipe, &buffer, bufferSize)
+            
+      if bytesRead > 0 {
+        let output = String(
+          bytes: buffer[0..<bytesRead],
+          encoding: .utf8
+        ) ?? ""
+                
+        let trimmed = output.trimmingCharacters(in: .newlines)
+        if !trimmed.isEmpty {
+          // we're sending stderr to oslog, so we need to avoid recursive calls
+          if trimmed.hasPrefix("OSLOG-") {
+            // make sure the system can parse the oslogs
+            write(writeToOriginal, &buffer, bytesRead)
+          } else {
+            Logger.info("[\(label)] \(trimmed)")
+          }
+        }
+      }
+    } 
+        
+    source.setCancelHandler {
+      close(readPipe)
+    }
+        
+    source.resume()
+    return source
+  }
+}
 
 /// Wrapper class for os_log function
 public class Logger {
@@ -21,7 +101,7 @@ public class Logger {
     }
   }
 
-  static func log(_ items: Any..., category: String, type: OSLogType) {
+  static func log(_ items: [Any], category: String, type: OSLogType) {
     if Logger.enabled {
       var message = ""
       let last = items.count - 1
@@ -31,6 +111,7 @@ public class Logger {
           message += " "
         }
       }
+
       let log = OSLog(subsystem: Bundle.main.bundleIdentifier ?? "-", category: category)
       os_log("%{public}@", log: log, type: type, String(message.prefix(4068)))
     }

crates/tauri/mobile/ios-api/Sources/Tauri/Tauri.swift

@@ -109,6 +109,14 @@ extension PluginManager: NSCopying {
   }
 }
 
+private var stdoutRedirector: StdoutRedirector?
+
+@_cdecl("log_stdout")
+func logStdout() {
+  stdoutRedirector = StdoutRedirector()
+  stdoutRedirector!.start()
+}
+
 @_cdecl("register_plugin")
 func registerPlugin(name: SRString, plugin: NSObject, config: SRString, webview: WKWebView?) {
   PluginManager.shared.load(

crates/tauri/src/ios.rs

@@ -46,3 +46,4 @@ swift!(pub fn register_plugin(
   webview: *const c_void
 ));
 swift!(pub fn on_webview_created(webview: *const c_void, controller: *const c_void));
+swift!(pub fn log_stdout());

crates/tauri/src/lib.rs

@@ -248,38 +248,10 @@ pub const VERSION: &str = env!("CARGO_PKG_VERSION");
 #[cfg(target_os = "ios")]
 #[doc(hidden)]
 pub fn log_stdout() {
-  use std::{
-    ffi::CString,
-    fs::File,
-    io::{BufRead, BufReader},
-    os::unix::prelude::*,
-    thread,
-  };
-
-  let mut logpipe: [RawFd; 2] = Default::default();
+  #[cfg(target_os = "ios")]
   unsafe {
-    libc::pipe(logpipe.as_mut_ptr());
-    libc::dup2(logpipe[1], libc::STDOUT_FILENO);
-    libc::dup2(logpipe[1], libc::STDERR_FILENO);
+    crate::ios::log_stdout();
   }
-  thread::spawn(move || unsafe {
-    let file = File::from_raw_fd(logpipe[0]);
-    let mut reader = BufReader::new(file);
-    let mut buffer = String::new();
-    loop {
-      buffer.clear();
-      if let Ok(len) = reader.read_line(&mut buffer) {
-        if len == 0 {
-          break;
-        } else if let Ok(msg) = CString::new(buffer.as_bytes())
-          .map_err(|_| ())
-          .and_then(|c| c.into_string().map_err(|_| ()))
-        {
-          log::info!("{}", msg);
-        }
-      }
-    }
-  });
 }
 
 /// The user event type.