feat: let a created buffer be filled through the safe API

src/core/buffer.rs

@@ -1,4 +1,4 @@
-use core::slice;
+use core::{ptr, slice};
 
 use crate::ffi::*;
 
@@ -67,6 +67,33 @@ pub trait MutableBuffer: Buffer {
         let buf = self.as_ngx_buf_mut();
         unsafe { slice::from_raw_parts_mut((*buf).pos, self.len()) }
     }
+
+    /// Returns how many bytes can still be appended.
+    ///
+    /// A buffer from [`crate::core::Pool::create_buffer`] starts empty, with
+    /// its whole allocation spare.
+    fn spare_capacity(&self) -> usize {
+        let buf = self.as_ngx_buf();
+        unsafe { usize::wrapping_sub((*buf).end as _, (*buf).last as _) }
+    }
+
+    /// Appends `bytes` to the buffer contents.
+    ///
+    /// Returns `None` and writes nothing if they do not fit, so a caller that
+    /// ignores the result cannot end up with a truncated value.
+    fn append(&mut self, bytes: &[u8]) -> Option<()> {
+        if bytes.len() > self.spare_capacity() {
+            return None;
+        }
+
+        let buf = self.as_ngx_buf_mut();
+        unsafe {
+            ptr::copy_nonoverlapping(bytes.as_ptr(), (*buf).last, bytes.len());
+            (*buf).last = (*buf).last.add(bytes.len());
+        }
+
+        Some(())
+    }
 }
 
 /// Wrapper struct for a temporary buffer, providing methods for working with an `ngx_buf_t`.
@@ -127,3 +154,50 @@ impl Buffer for MemoryBuffer {
         self.0
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// Builds the shape `ngx_create_temp_buf` produces: `pos == last == start`.
+    fn temp_buf(storage: &mut [u8], raw: &mut ngx_buf_t) -> TemporaryBuffer {
+        raw.start = storage.as_mut_ptr();
+        raw.pos = raw.start;
+        raw.last = raw.start;
+        raw.end = unsafe { raw.start.add(storage.len()) };
+        TemporaryBuffer::from_ngx_buf(raw)
+    }
+
+    #[test]
+    fn append_fills_a_freshly_created_buffer() {
+        let mut storage = [0u8; 64];
+        let mut raw: ngx_buf_t = unsafe { core::mem::zeroed() };
+        let mut buf = temp_buf(&mut storage, &mut raw);
+
+        // A new buffer holds nothing, and all of its allocation is spare.
+        assert_eq!(buf.len(), 0);
+        assert_eq!(buf.spare_capacity(), 64);
+        assert!(buf.as_bytes_mut().is_empty());
+
+        assert!(buf.append(b"hello ").is_some());
+        assert!(buf.append(b"world").is_some());
+
+        assert_eq!(buf.as_bytes(), b"hello world");
+        assert_eq!(buf.len(), 11);
+        assert_eq!(buf.spare_capacity(), 53);
+    }
+
+    #[test]
+    fn append_writes_nothing_when_it_does_not_fit() {
+        let mut storage = [0u8; 4];
+        let mut raw: ngx_buf_t = unsafe { core::mem::zeroed() };
+        let mut buf = temp_buf(&mut storage, &mut raw);
+
+        assert!(buf.append(b"too long").is_none());
+        assert_eq!(buf.len(), 0);
+
+        // A partial append would have left the buffer holding "too ".
+        assert!(buf.append(b"ok").is_some());
+        assert_eq!(buf.as_bytes(), b"ok");
+    }
+}