merge-file: add --diff-algorithm option

Make it possible to use other diff algorithms than the 'myers'
default algorithm, when using the 'git merge-file' command, to help
avoid spurious conflicts by selecting a more recent algorithm such
as 'histogram', for instance when using 'git merge-file' as part of
a custom merge driver.

Signed-off-by: Antonin Delpeuch <antonin@delpeuch.eu>
Reviewed-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/Documentation/git-merge-file.txt b/Documentation/git-merge-file.txt
index 6a081eacb72372e332850118ab9df7280fa6f08d..71915a00fa472e73f5fadd2205f964919d3262db 100644 (file)
--- a/Documentation/git-merge-file.txt
+++ b/Documentation/git-merge-file.txt
@@ -92,6 +92,12 @@ object store and the object ID of its blob is written to standard output.
        Instead of leaving conflicts in the file, resolve conflicts
        favouring our (or their or both) side of the lines.
 
+--diff-algorithm={patience|minimal|histogram|myers}::
+       Use a different diff algorithm while merging. The current default is "myers",
+       but selecting more recent algorithm such as "histogram" can help
+       avoid mismerges that occur due to unimportant matching lines
+       (such as braces from distinct functions). See also
+       linkgit:git-diff[1] `--diff-algorithm`.
 
 EXAMPLES
 --------

diff --git a/builtin/merge-file.c b/builtin/merge-file.c
index 832c93d8d54cc50b8581102be4db7c92d80e738a..1f987334a31b0e19904825297c1bb0d37a1fc2fe 100644 (file)
--- a/builtin/merge-file.c
+++ b/builtin/merge-file.c
@@ -1,5 +1,6 @@
 #include "builtin.h"
 #include "abspath.h"
+#include "diff.h"
 #include "hex.h"
 #include "object-name.h"
 #include "object-store.h"
@@ -28,6 +29,30 @@ static int label_cb(const struct option *opt, const char *arg, int unset)
        return 0;
 }
 
+static int set_diff_algorithm(xpparam_t *xpp,
+                             const char *alg)
+{
+       long diff_algorithm = parse_algorithm_value(alg);
+       if (diff_algorithm < 0)
+               return -1;
+       xpp->flags = (xpp->flags & ~XDF_DIFF_ALGORITHM_MASK) | diff_algorithm;
+       return 0;
+}
+
+static int diff_algorithm_cb(const struct option *opt,
+                               const char *arg, int unset)
+{
+       xpparam_t *xpp = opt->value;
+
+       BUG_ON_OPT_NEG(unset);
+
+       if (set_diff_algorithm(xpp, arg))
+               return error(_("option diff-algorithm accepts \"myers\", "
+                              "\"minimal\", \"patience\" and \"histogram\""));
+
+       return 0;
+}
+
 int cmd_merge_file(int argc, const char **argv, const char *prefix)
 {
        const char *names[3] = { 0 };
@@ -48,6 +73,9 @@ int cmd_merge_file(int argc, const char **argv, const char *prefix)
                            XDL_MERGE_FAVOR_THEIRS),
                OPT_SET_INT(0, "union", &xmp.favor, N_("for conflicts, use a union version"),
                            XDL_MERGE_FAVOR_UNION),
+               OPT_CALLBACK_F(0, "diff-algorithm", &xmp.xpp, N_("<algorithm>"),
+                            N_("choose a diff algorithm"),
+                            PARSE_OPT_NONEG, diff_algorithm_cb),
                OPT_INTEGER(0, "marker-size", &xmp.marker_size,
                            N_("for conflicts, use this marker size")),
                OPT__QUIET(&quiet, N_("do not warn about conflicts")),

diff --git a/t/t6403-merge-file.sh b/t/t6403-merge-file.sh
index 2c92209ecabf5b17477522a328e64ac0c5e62ee1..fb872c5a1136fc523dacdf6f9778b37441905980 100755 (executable)
--- a/t/t6403-merge-file.sh
+++ b/t/t6403-merge-file.sh
@@ -56,7 +56,67 @@ test_expect_success 'setup' '
        deduxit me super semitas jusitiae,
        EOF
 
-       printf "propter nomen suum." >>new4.txt
+       printf "propter nomen suum." >>new4.txt &&
+
+       cat >base.c <<-\EOF &&
+       int f(int x, int y)
+       {
+               if (x == 0)
+               {
+                       return y;
+               }
+               return x;
+       }
+
+       int g(size_t u)
+       {
+               while (u < 30)
+               {
+                       u++;
+               }
+               return u;
+       }
+       EOF
+
+       cat >ours.c <<-\EOF &&
+       int g(size_t u)
+       {
+               while (u < 30)
+               {
+                       u++;
+               }
+               return u;
+       }
+
+       int h(int x, int y, int z)
+       {
+               if (z == 0)
+               {
+                       return x;
+               }
+               return y;
+       }
+       EOF
+
+       cat >theirs.c <<-\EOF
+       int f(int x, int y)
+       {
+               if (x == 0)
+               {
+                       return y;
+               }
+               return x;
+       }
+
+       int g(size_t u)
+       {
+               while (u > 34)
+               {
+                       u--;
+               }
+               return u;
+       }
+       EOF
 '
 
 test_expect_success 'merge with no changes' '
@@ -447,4 +507,66 @@ test_expect_success '--object-id fails without repository' '
        grep "not a git repository" err
 '
 
+test_expect_success 'merging C files with "myers" diff algorithm creates some spurious conflicts' '
+       cat >expect.c <<-\EOF &&
+       int g(size_t u)
+       {
+               while (u < 30)
+               {
+                       u++;
+               }
+               return u;
+       }
+
+       int h(int x, int y, int z)
+       {
+       <<<<<<< ours.c
+               if (z == 0)
+       ||||||| base.c
+               while (u < 30)
+       =======
+               while (u > 34)
+       >>>>>>> theirs.c
+               {
+       <<<<<<< ours.c
+                       return x;
+       ||||||| base.c
+                       u++;
+       =======
+                       u--;
+       >>>>>>> theirs.c
+               }
+               return y;
+       }
+       EOF
+
+       test_must_fail git merge-file -p --diff3 --diff-algorithm myers ours.c base.c theirs.c >myers_output.c &&
+       test_cmp expect.c myers_output.c
+'
+
+test_expect_success 'merging C files with "histogram" diff algorithm avoids some spurious conflicts' '
+       cat >expect.c <<-\EOF &&
+       int g(size_t u)
+       {
+               while (u > 34)
+               {
+                       u--;
+               }
+               return u;
+       }
+
+       int h(int x, int y, int z)
+       {
+               if (z == 0)
+               {
+                       return x;
+               }
+               return y;
+       }
+       EOF
+
+       git merge-file -p --diff3 --diff-algorithm histogram ours.c base.c theirs.c >histogram_output.c &&
+       test_cmp expect.c histogram_output.c
+'
+
 test_done