feat: add a buffering feature to AdminLog

Signed-off-by: Lars Wirzenius <liw@liw.fi>

src/msg.rs

@@ -1432,6 +1432,7 @@ pub mod helper {
         filename: Option<PathBuf>,
         file: Option<File>,
         stderr: bool,
+        buffer: Option<Vec<u8>>,
     }
 
     impl AdminLog {
@@ -1447,9 +1448,25 @@ pub mod helper {
                 filename: None,
                 file: None,
                 stderr: true,
+                buffer: None,
             }
         }
 
+        /// Capture output into a buffer.
+        pub fn capture() -> Self {
+            Self {
+                filename: None,
+                file: None,
+                stderr: false,
+                buffer: Some(vec![]),
+            }
+        }
+
+        /// Return current buffer created by `capture`.
+        pub fn capture_buffer(&self) -> Option<&[u8]> {
+            self.buffer.as_deref()
+        }
+
         /// Create an admin log that writes to a named file.
         pub fn open(filename: &Path) -> Result<Self, LogError> {
             let file = OpenOptions::new()
@@ -1461,6 +1478,7 @@ pub mod helper {
                 filename: Some(filename.into()),
                 file: Some(file),
                 stderr: false,
+                buffer: None,
             })
         }
 
@@ -1483,6 +1501,8 @@ pub mod helper {
                 std::io::stderr()
                     .write_all(msg.as_bytes())
                     .map_err(LogError::WriteLogStderr)?;
+            } else if let Some(buf) = self.buffer.as_mut() {
+                buf.extend_from_slice(msg.as_bytes());
             }
             Ok(())
         }