build: surface a panicked output-reader thread

run_command_capturing discarded the pump threads' join results: a
reader that died mid-capture (UI sink or log writer failing) reported a
successful build with truncated captured logs. A reader panic now fails
the command; when the child itself failed first, its error keeps
precedence and the reader panic is logged so the truncated output is
not silently lost.

src/build/mod.rs

@@ -815,7 +815,9 @@ struct CommandFailure {
 /// it (live view + tee log) while the stderr is additionally captured for
 /// the failure classification; otherwise stdio is inherited from the
 /// terminal. Returns an error (with the captured stderr) on non-zero exit
-/// status.
+/// status. A panicking stdout/stderr reader thread also yields an error
+/// (the captured output would be incomplete), but only after the command's
+/// own failure, which takes precedence.
 fn run_command_capturing(
     cwd: &Path,
     program: &str,
@@ -837,6 +839,8 @@ fn run_command_capturing(
     // classification.
     let stderr_capture = Arc::new(std::sync::Mutex::new(String::new()));
 
+    // Printable panic message from a reader thread, if one died mid-pump.
+    let mut reader_panic = None;
     let status = match sink {
         None => cmd.status().map_err(|e| CommandFailure {
             error: format!("failed to run '{}': {}", program, e).into(),
@@ -873,8 +877,11 @@ fn run_command_capturing(
                     );
                 }
             });
-            let _ = out_thread.join();
-            let _ = err_thread.join();
+            // The threads end on EOF, i.e. once the child exited and closed
+            // its streams; a panic from either means the captured output is
+            // incomplete.
+            reader_panic = reader_panic_message(out_thread.join())
+                .or_else(|| reader_panic_message(err_thread.join()));
 
             child.wait().map_err(|e| CommandFailure {
                 error: format!("failed to wait for '{}': {}", program, e).into(),
@@ -884,6 +891,11 @@ fn run_command_capturing(
     };
 
     if !status.success() {
+        // The command's own failure takes precedence over a dead reader; the
+        // panic is still logged so the truncated-log cause is not lost.
+        if let Some(message) = &reader_panic {
+            log::error!("the build output reader failed: {message}");
+        }
         return Err(CommandFailure {
             error: format!(
                 "'{} {}' failed with status: {}",
@@ -895,9 +907,30 @@ fn run_command_capturing(
             stderr: std::mem::take(&mut stderr_capture.lock().unwrap_or_else(|e| e.into_inner())),
         });
     }
+
+    // The command succeeded but a reader thread panicked: the captured output
+    // (live view + tee log) is incomplete, so this cannot pass as a success.
+    if let Some(message) = reader_panic {
+        return Err(CommandFailure {
+            error: format!("the build output reader failed: {message}").into(),
+            stderr: std::mem::take(&mut stderr_capture.lock().unwrap_or_else(|e| e.into_inner())),
+        });
+    }
     Ok(())
 }
 
+/// Extract a printable message from a reader-thread join result; `None` when
+/// the thread finished normally.
+fn reader_panic_message(join: std::thread::Result<()>) -> Option<String> {
+    join.err().map(|payload| {
+        payload
+            .downcast_ref::<&str>()
+            .map(|s| (*s).to_string())
+            .or_else(|| payload.downcast_ref::<String>().cloned())
+            .unwrap_or_else(|| "non-string panic payload".to_string())
+    })
+}
+
 /// Run a build command, discarding the captured stderr.
 fn run_command(
     cwd: &Path,