diff --git a/contrib/libdiff/include/diff_main.h b/contrib/libdiff/include/diff_main.h
--- a/contrib/libdiff/include/diff_main.h
+++ b/contrib/libdiff/include/diff_main.h
@@ -118,6 +118,7 @@
 
 /* Flags set by file atomizer. */
 #define DIFF_ATOMIZER_FOUND_BINARY_DATA	0x00000001
+#define DIFF_ATOMIZER_FILE_TRUNCATED	0x00000002
 
 /* Flags set by caller of diff_main(). */
 #define DIFF_FLAG_IGNORE_WHITESPACE	0x00000001
diff --git a/contrib/libdiff/lib/diff_atomize_text.c b/contrib/libdiff/lib/diff_atomize_text.c
--- a/contrib/libdiff/lib/diff_atomize_text.c
+++ b/contrib/libdiff/lib/diff_atomize_text.c
@@ -16,6 +16,8 @@
  */
 
 #include <errno.h>
+#include <setjmp.h>
+#include <signal.h>
 #include <stdbool.h>
 #include <stdint.h>
 #include <stdio.h>
@@ -122,10 +124,18 @@
 	return DIFF_RC_OK;
 }
 
+static sigjmp_buf diff_data_signal_env;
+static void
+diff_data_signal_handler(int sig)
+{
+	siglongjmp(diff_data_signal_env, sig);
+}
+
 static int
 diff_data_atomize_text_lines_mmap(struct diff_data *d)
 {
-	const uint8_t *pos = d->data;
+	struct sigaction act, oact;
+	const uint8_t *volatile pos = d->data;
 	const uint8_t *end = pos + d->len;
 	bool ignore_whitespace = (d->diff_flags & DIFF_FLAG_IGNORE_WHITESPACE);
 	bool embedded_nul = false;
@@ -136,8 +146,22 @@
 
 	ARRAYLIST_INIT(d->atoms, 1 << pow2);
 
+	sigemptyset(&act.sa_mask);
+	act.sa_flags = 0;
+	act.sa_handler = diff_data_signal_handler;
+	sigaction(SIGBUS, &act, &oact);
+	if (sigsetjmp(diff_data_signal_env, 0) > 0) {
+		/*
+		 * The file was truncated while we were reading it.  Set
+		 * the end pointer to the beginning of the line we were
+		 * trying to read, adjust the file length, and set a flag.
+		 */
+		end = pos;
+		d->len = end - d->data;
+		d->atomizer_flags |= DIFF_ATOMIZER_FILE_TRUNCATED;
+	}
 	while (pos < end) {
-		const uint8_t *line_end = pos;
+		const uint8_t *line_start = pos, *line_end = pos;
 		unsigned int hash = 0;
 
 		while (line_end < end && *line_end != '\r' && *line_end != '\n') {
@@ -164,15 +188,16 @@
 
 		*atom = (struct diff_atom){
 			.root = d,
-			.pos = (off_t)(pos - d->data),
-			.at = pos,
-			.len = line_end - pos,
+			.pos = (off_t)(line_start - d->data),
+			.at = line_start,
+			.len = line_end - line_start,
 			.hash = hash,
 		};
 
 		/* Starting point for next line: */
 		pos = line_end;
 	}
+	sigaction(SIGBUS, &oact, NULL);
 
 	/* File are considered binary if they contain embedded '\0' bytes. */
 	if (embedded_nul)