rust/qemu-api: log: implement io::Write

This makes it possible to lock the log file; it also makes log_mask_ln!
not allocate memory when logging a constant string.

Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Reviewed-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/qemu/log.h

@@ -84,6 +84,8 @@ typedef struct QEMULogItem {
 
 extern const QEMULogItem qemu_log_items[];
 
+ssize_t rust_fwrite(const void *ptr, size_t size, size_t nmemb, FILE *stream);
+
 bool qemu_set_log(int log_flags, Error **errp);
 bool qemu_set_log_filename(const char *filename, Error **errp);
 bool qemu_set_log_filename_flags(const char *name, int flags, Error **errp);

rust/qemu-api/src/log.rs

@@ -3,6 +3,13 @@
 
 //! Bindings for QEMU's logging infrastructure
 
+use std::{
+    io::{self, Write},
+    ptr::NonNull,
+};
+
+use crate::{bindings, errno};
+
 #[repr(u32)]
 /// Represents specific error categories within QEMU's logging system.
 ///
@@ -11,11 +18,82 @@
 pub enum Log {
     /// Log invalid access caused by the guest.
     /// Corresponds to `LOG_GUEST_ERROR` in the C implementation.
-    GuestError = crate::bindings::LOG_GUEST_ERROR,
+    GuestError = bindings::LOG_GUEST_ERROR,
 
     /// Log guest access of unimplemented functionality.
     /// Corresponds to `LOG_UNIMP` in the C implementation.
-    Unimp = crate::bindings::LOG_UNIMP,
+    Unimp = bindings::LOG_UNIMP,
+}
+
+/// A RAII guard for QEMU's logging infrastructure.  Creating the guard
+/// locks the log file, and dropping it (letting it go out of scope) unlocks
+/// the file.
+///
+/// As long as the guard lives, it can be written to using [`std::io::Write`].
+///
+/// The locking is recursive, therefore owning a guard does not prevent
+/// using [`log_mask_ln!()`](crate::log_mask_ln).
+pub struct LogGuard(NonNull<bindings::FILE>);
+
+impl LogGuard {
+    /// Return a RAII guard that writes to QEMU's logging infrastructure.
+    /// The log file is locked while the guard exists, ensuring that there
+    /// is no tearing of the messages.
+    ///
+    /// Return `None` if the log file is closed and could not be opened.
+    /// Do *not* use `unwrap()` on the result; failure can be handled simply
+    /// by not logging anything.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use qemu_api::log::LogGuard;
+    /// # use std::io::Write;
+    /// if let Some(mut log) = LogGuard::new() {
+    ///     writeln!(log, "test");
+    /// }
+    /// ```
+    pub fn new() -> Option<Self> {
+        let f = unsafe { bindings::qemu_log_trylock() }.cast();
+        NonNull::new(f).map(Self)
+    }
+
+    /// Writes a formatted string into the log, returning any error encountered.
+    ///
+    /// This method is primarily used by the
+    /// [`log_mask_ln!()`](crate::log_mask_ln) macro, and it is rare for it
+    /// to be called explicitly.  It is public because it is the only way to
+    /// examine the error, which `log_mask_ln!()` ignores
+    ///
+    /// Unlike `log_mask_ln!()`, it does *not* append a newline at the end.
+    pub fn log_fmt(args: std::fmt::Arguments) -> io::Result<()> {
+        if let Some(mut log) = Self::new() {
+            log.write_fmt(args)?;
+        }
+        Ok(())
+    }
+}
+
+impl Write for LogGuard {
+    fn write(&mut self, bytes: &[u8]) -> io::Result<usize> {
+        let ret = unsafe {
+            bindings::rust_fwrite(bytes.as_ptr().cast(), 1, bytes.len(), self.0.as_ptr())
+        };
+        errno::into_io_result(ret)
+    }
+
+    fn flush(&mut self) -> io::Result<()> {
+        // Do nothing, dropping the guard takes care of flushing
+        Ok(())
+    }
+}
+
+impl Drop for LogGuard {
+    fn drop(&mut self) {
+        unsafe {
+            bindings::qemu_log_unlock(self.0.as_ptr());
+        }
+    }
 }
 
 /// A macro to log messages conditionally based on a provided mask.
@@ -24,6 +102,8 @@ pub enum Log {
 /// log level and, if so, formats and logs the message. It is the Rust
 /// counterpart of the `qemu_log_mask()` macro in the C implementation.
 ///
+/// Errors from writing to the log are ignored.
+///
 /// # Parameters
 ///
 /// - `$mask`: A log level mask. This should be a variant of the `Log` enum.
@@ -62,12 +142,8 @@ macro_rules! log_mask_ln {
         if unsafe {
             (::qemu_api::bindings::qemu_loglevel & ($mask as std::os::raw::c_int)) != 0
         } {
-            let formatted_string = format!("{}\n", format_args!($fmt $($args)*));
-            let c_string = std::ffi::CString::new(formatted_string).unwrap();
-
-            unsafe {
-                ::qemu_api::bindings::qemu_log(c_string.as_ptr());
-            }
+            _ = ::qemu_api::log::LogGuard::log_fmt(
+                format_args!("{}\n", format_args!($fmt $($args)*)));
         }
     }};
 }

util/log.c

@@ -558,3 +558,15 @@ void qemu_print_log_usage(FILE *f)
     fprintf(f, "\nUse \"-d trace:help\" to get a list of trace events.\n\n");
 #endif
 }
+
+#ifdef CONFIG_HAVE_RUST
+ssize_t rust_fwrite(const void *ptr, size_t size, size_t nmemb, FILE *stream)
+{
+    /*
+     * Same as fwrite, but return -errno because Rust libc does not provide
+     * portable access to errno. :(
+     */
+    int ret = fwrite(ptr, size, nmemb, stream);
+    return ret < 0 ? -errno : 0;
+}
+#endif