Auto merge of #21812 - jdm:useful-output, r=SimonSapin

Provide useful defaults for Android logcat output

These changes integrate the stdout/stderr redirection from 0e80cfbcee/cargo-apk/injected-glue/lib.rs (L240-L303), and set up the default RUST_LOG filters so that useful JS errors and GL errors appear in the logcat output. I have verified that thread panics appear in output as well (without backtraces, sadly) by visiting https://acid3.acidtests.org.

---
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #21637 and fix #21783
- [x] These changes do not require tests because no tests for logcat.

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ports/libsimpleservo/src/jniapi.rs

@@ -10,11 +10,13 @@ use gl_glue;
 use jni::{JNIEnv, JavaVM};
 use jni::objects::{GlobalRef, JClass, JObject, JString, JValue};
 use jni::sys::{jboolean, jfloat, jint, jstring, JNI_TRUE};
+use libc::{pipe, dup2, read};
 use log::Level;
 use std;
 use std::borrow::Cow;
-use std::os::raw::c_void;
+use std::os::raw::{c_char, c_int, c_void};
 use std::sync::{Arc, Mutex};
+use std::thread;
 
 struct HostCallbacks {
     callbacks: GlobalRef,
@@ -61,11 +63,19 @@ pub fn Java_com_mozilla_servoview_JNIServo_init(
         // Note: Android debug logs are stripped from a release build.
         // debug!() will only show in a debug build. Use info!() if logs
         // should show up in adb logcat with a release build.
-        let mut filter = Filter::default()
-            .with_min_level(Level::Debug)
-            .with_allowed_module_path("simpleservo::api")
-            .with_allowed_module_path("simpleservo::jniapi")
-            .with_allowed_module_path("simpleservo::gl_glue::egl");
+        let filters = [
+            "simpleservo::api",
+            "simpleservo::jniapi",
+            "simpleservo::gl_glue::egl",
+            // Show JS errors by default.
+            "script::dom::bindings::error",
+            // Show GL errors by default.
+            "canvas::webgl_thread",
+        ];
+        let mut filter = Filter::default().with_min_level(Level::Debug);
+        for &module in &filters {
+            filter = filter.with_allowed_module_path(module);
+        }
         let log_str = env.get_string(log_str).ok();
         let log_str = log_str.as_ref().map_or(Cow::Borrowed(""), |s| s.to_string_lossy());
         for module in log_str.split(',') {
@@ -77,6 +87,7 @@ pub fn Java_com_mozilla_servoview_JNIServo_init(
     info!("init");
 
     initialize_android_glue(&env, activity);
+    redirect_stdout_to_logcat();
 
     let args = env.get_string(args)
         .expect("Couldn't get java string")
@@ -434,3 +445,78 @@ fn initialize_android_glue(env: &JNIEnv, activity: JObject) {
         ANDROID_APP = Box::into_raw(Box::new(app));
     }
 }
+
+extern {
+    pub fn __android_log_write(prio: c_int, tag: *const c_char, text: *const c_char) -> c_int;
+}
+
+fn redirect_stdout_to_logcat() {
+    // The first step is to redirect stdout and stderr to the logs.
+    // We redirect stdout and stderr to a custom descriptor.
+    let mut pfd: [c_int; 2] = [0, 0];
+    unsafe {
+        pipe(pfd.as_mut_ptr());
+        dup2(pfd[1], 1);
+        dup2(pfd[1], 2);
+    }
+
+    let descriptor = pfd[0];
+
+    // Then we spawn a thread whose only job is to read from the other side of the
+    // pipe and redirect to the logs.
+    let _detached = thread::spawn(move || {
+        const BUF_LENGTH: usize = 512;
+        let mut buf = vec![b'\0' as c_char; BUF_LENGTH];
+
+        // Always keep at least one null terminator
+        const BUF_AVAILABLE: usize = BUF_LENGTH - 1;
+        let buf = &mut buf[..BUF_AVAILABLE];
+
+        let mut cursor = 0_usize;
+
+        let tag = b"simpleservo\0".as_ptr() as _;
+
+        loop {
+            let result = {
+                let read_into = &mut buf[cursor..];
+                unsafe {
+                    read(descriptor, read_into.as_mut_ptr() as *mut _, read_into.len())
+                }
+            };
+
+            let end = if result == 0 {
+                return;
+            } else if result < 0 {
+                return; /* TODO: report problem */
+            } else {
+                result as usize + cursor
+            };
+
+            if let Some(last_newline_pos) = buf.iter().rposition(|&c| c == b'\n' as c_char) {
+                buf[last_newline_pos] = b'\0' as c_char;
+                unsafe {
+                    __android_log_write(3, tag, buf.as_ptr());
+                }
+                if last_newline_pos < buf.len() - 1 {
+                    let pos_after_newline = last_newline_pos + 1;
+                    let len_not_logged_yet = buf[pos_after_newline..].len();
+                    for j in 0..len_not_logged_yet as usize {
+                        buf[j] = buf[pos_after_newline + j];
+                    }
+                    cursor = len_not_logged_yet;
+                } else {
+                    cursor = 0;
+                }
+            } else if cursor == BUF_AVAILABLE {
+                // No newline found but the buffer is full, flush it anyway.
+                // `buf.as_ptr()` is null-terminated by BUF_LENGTH being 1 less than BUF_AVAILABLE.
+                unsafe {
+                    __android_log_write(3, tag, buf.as_ptr());
+                }
+                cursor = 0;
+            } else {
+                cursor = end;
+            }
+        }
+    });
+}