xdiff: avoid arithmetic overflow in xdl_get_hunk()

xdl_get_hunk() calculates the maximum number of common lines between two
changes that would fit into the same hunk for the given context options.
It involves doubling and addition and thus can overflow if the terms are
huge.

The type of ctxlen and interhunkctxlen in xdemitconf_t is long, while
the type of the corresponding context and interhunkcontext in struct
diff_options is int.  On many platforms longs are bigger that ints,
which prevents the overflow.  On Windows they have the same range and
the overflow manifests as hunks that are split erroneously and lines
being repeated between them.

Fix the overflow by checking and not going beyond LONG_MAX.  This allows
specifying a huge context line count and getting all lines of a changed
files in a single hunk, as expected.

Reported-by: Jason Cho <jason11choca@proton.me>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: rscharfe

t/t4055-diff-context.sh

@@ -90,4 +90,14 @@ test_expect_success '-U0 is valid, so is diff.context=0' '
 	grep "^+MODIFIED" output
 '
 
+test_expect_success '-U2147483647 works' '
+	echo APPENDED >>x &&
+	test_line_count = 16 x &&
+	git diff -U2147483647 >output &&
+	test_line_count = 22 output &&
+	grep "^-ADDED" output &&
+	grep "^+MODIFIED" output &&
+	grep "^+APPENDED" output
+'
+
 test_done

xdiff/xemit.c

@@ -43,6 +43,10 @@ static int xdl_emit_record(xdfile_t *xdf, long ri, char const *pre, xdemitcb_t *
 	return 0;
 }
 
+static long saturating_add(long a, long b)
+{
+	return signed_add_overflows(a, b) ? LONG_MAX : a + b;
+}
 
 /*
  * Starting at the passed change atom, find the latest change atom to be included
@@ -52,7 +56,9 @@ static int xdl_emit_record(xdfile_t *xdf, long ri, char const *pre, xdemitcb_t *
 xdchange_t *xdl_get_hunk(xdchange_t **xscr, xdemitconf_t const *xecfg)
 {
 	xdchange_t *xch, *xchp, *lxch;
-	long max_common = 2 * xecfg->ctxlen + xecfg->interhunkctxlen;
+	long max_common = saturating_add(saturating_add(xecfg->ctxlen,
+							xecfg->ctxlen),
+					 xecfg->interhunkctxlen);
 	long max_ignorable = xecfg->ctxlen;
 	unsigned long ignored = 0; /* number of ignored blank lines */