grep: -B retains exactly N context lines, not next power of two

src/context_buffer.rs

@@ -42,13 +42,17 @@ impl BufferedLine {
 /// provide a more optimized `ContextBuffer` when `mmap()` is available.
 pub struct ContextBuffer {
     slots: Vec<BufferedLine>,
+    /// Number of lines to retain — the requested context size. The slot count is
+    /// rounded up to a power of two for cheap `& mask` indexing, so it must not be
+    /// used as the retention limit.
+    capacity: usize,
     head: usize,
     len: usize,
 }
 
 impl ContextBuffer {
     pub fn new(capacity: usize) -> Self {
-        let len = if capacity == 0 {
+        let slots = if capacity == 0 {
             0
         } else {
             capacity.next_power_of_two()
@@ -60,8 +64,9 @@ impl ContextBuffer {
                     line_number: 0,
                     byte_offset: 0,
                 };
-                len
+                slots
             ],
+            capacity,
             head: 0,
             len: 0,
         }
@@ -90,7 +95,7 @@ impl ContextBuffer {
         slot.byte_offset = byte_offset;
 
         self.head = self.head.wrapping_add(1);
-        self.len = (self.len + 1).min(self.slots.len());
+        self.len = (self.len + 1).min(self.capacity);
     }
 
     pub fn drain_iter(&mut self) -> impl Iterator<Item = &BufferedLine> {

tests/test_grep.rs

@@ -909,6 +909,45 @@ fn after_before_combined_context() {
         .stdout_only("b\nMATCH\nc\n");
 }
 
+#[test]
+fn before_context_count_is_exact_not_rounded() {
+    // `-B N` must retain exactly N preceding lines. The ring buffer rounds its
+    // slot count up to a power of two, so a non-power-of-two N (3, 5, 6, 7) must
+    // not leak extra context lines.
+    let input = "01\n02\n03\n04\n05\n06\n07\n08\n09\n10\nMM\n";
+
+    let expected = [
+        ("0", "MM\n"),
+        ("1", "10\nMM\n"),
+        ("2", "09\n10\nMM\n"),
+        ("3", "08\n09\n10\nMM\n"),
+        ("4", "07\n08\n09\n10\nMM\n"),
+        ("5", "06\n07\n08\n09\n10\nMM\n"),
+        ("6", "05\n06\n07\n08\n09\n10\nMM\n"),
+        ("7", "04\n05\n06\n07\n08\n09\n10\nMM\n"),
+    ];
+    for (n, want) in expected {
+        let (_s, mut c) = ucmd();
+        c.args(&["-B", n, "MM"])
+            .pipe_in(input)
+            .succeeds()
+            .stdout_only(want);
+    }
+
+    // A count larger than the available lines just caps at what is there.
+    let (_s, mut c) = ucmd();
+    c.args(&["-B", "1000000", "MM"])
+        .pipe_in(input)
+        .succeeds()
+        .stdout_only(input);
+
+    // Non-numeric, negative, and fractional counts are rejected with exit code 2.
+    for bad in ["abc", "-1", "2.5"] {
+        let (_s, mut c) = ucmd();
+        c.args(&["-B", bad, "MM"]).pipe_in(input).fails_with_code(2);
+    }
+}
+
 #[test]
 fn num_shorthand_is_context() {
     // `-2` is shorthand for `-C 2`.