Rewrite android logging to use checked slices.

ports/libsimpleservo/src/jniapi.rs

@@ -14,7 +14,6 @@ use libc::{pipe, dup2, read};
 use log::Level;
 use std;
 use std::borrow::Cow;
-use std::ffi::CString;
 use std::os::raw::{c_char, c_int, c_void};
 use std::sync::{Arc, Mutex};
 use std::thread;
@@ -466,51 +465,57 @@ fn redirect_stdout_to_logcat() {
     // 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 || {
-        unsafe {
-            let mut buf: Vec<c_char> = Vec::with_capacity(512);
-            let mut cursor = 0_usize;
-
-            // TODO: shouldn't use Rust stdlib
-            let tag = CString::new("simpleservo").unwrap();
-            let tag = tag.as_ptr();
-
-            loop {
-                let result = read(descriptor, buf.as_mut_ptr().offset(cursor as isize) as *mut _,
-                                  buf.capacity() - 1 - cursor);
-
-                let len = if result == 0 {
-                    return;
-                } else if result < 0 {
-                    return; /* TODO: report problem */
-                } else {
-                    result as usize + cursor
-                };
+        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];
 
-                buf.set_len(len);
+        let mut cursor = 0_usize;
 
-                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;
+        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 last_newline_pos = last_newline_pos + 1;
-                        cursor = buf.len() - last_newline_pos;
-                        debug_assert!(cursor < buf.capacity());
-                        for j in 0..cursor as usize {
-                            buf[j] = buf[last_newline_pos + j];
-                        }
-                        buf[cursor] = b'\0' as c_char;
-                        buf.set_len(cursor + 1);
-                    } else {
-                        cursor = 0;
+                }
+                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 = buf.len();
+                    cursor = 0;
                 }
-                if cursor == buf.capacity() - 1 {
+            } 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());
-                    buf.set_len(0);
-                    cursor = 0;
                 }
+                cursor = 0;
+            } else {
+                cursor = end;
             }
         }
     });