git还会使用3路合并吗？ [英] Does git revert also use the 3-way-merge?
问题描述
运行 git revert 时，可能会发生冲突。 git是否依赖于3-way-merge，正如 p>
回复的合并基础是什么？在使用git和meld进行交互式重新绑定的三种合并方式中的三个文件是什么？这很清楚，但很难想象这种方式会被还原。
A - B - C - D - C ^ -1
（如果我想在最后还原 C ）。 >
是的，有一个基地。 （注意：自从我多年前看了这段代码后，这段代码已经发生了很大的变化，我选择了一些关于我最近挑选的答案，你已经链接到这里了。）
git cherry-pick 和 git revert 由相同的源文件（ builtin / revert.c 和 sequencer.c ）。 说，棘手的部分是决定什么假合并基地。在你的例子中，我们正在撤消 B --to - C diffs。下面是实际的源代码（在 sequencer.c 中），稍微剥离了一下：
if（opts-> action == REPLAY_REVERT）{
ase = commit;
base_label = msg.label;
next = parent;
next_label = msg.parent_label;
strbuf_addstr（& msgbuf，Revert \）;
strbuf_addstr（& msgbuf，msg.subject）;
strbuf_addstr（& msgbuf，\\\\
\\\
This恢复提交）;
strbuf_addstr（& msgbuf，oid_to_hex（& commit-> object.oid））;
if（commit-> parents&& commit-> parents-> next）{
strbuf_addstr（& msgbuf，reverseing \\\
changes made to） ;
strbuf_addstr（& msgbuf，oid_to_hex（& parent-> object.oid））;
}
strbuf_addstr（& msgbuf，.\）;
}其他{
[这是樱桃挑选的案例，仅供参考）
const char * p;
base = parent;
base_label = msg.parent_label;
next = commit;
next_label = msg.label;
当我们在这里输入时， commit 指向 C 和父母的数据指向 B 的数据。对变量 base 的赋值是设置合并基的内容，并且 next -vs - base 是带入的内容。对于樱桃挑选，提交的父项（可能通过 -m 选择）是合并基础。为了回复，提交本身就是合并基础，并且父母（可能从 -m ）可以带入。
获得相同效果的另一种方式（多年前是这样做的，直到最近，我还认为它仍在使用中）是反向应用由 git format-patch 。在这种情况下，构建的基本版本是第二个散列（来自 A..B 部分的 B 部分）一个文本差异）：
/ *
*这表示一个文件的补丁，元信息变化为
*如创建/删除，文件模式和内容更改以
*表示为一系列片段。
* /
struct patch {
[snip]
char old_sha1_prefix [41];
char new_sha1_prefix [41];
static void reverse_patches（struct patch * p）
{
[snip]
swap（p-> old_sha1_prefix，p-> new_sha1_prefix）;
索引行中提取哈希的代码之后，将 A 和 B 部分放入旧的和新的前缀字段中。然后（在 reverse_patches 之后），当实际应用每个补丁时，git使用保存的旧的和新的sha1值来伪造3路合并（如果 git am 给出 - 3way ）。因此，通过反向应用文本补丁，我们将获取新文件作为基准，原始文件作为目标，就像 sequencer.c 代码一样。
When I run git revert, it can happen, that a conflict occurs. Does git rely on the 3-way-merge, as it is depicted in the question merge internals (cf. table below) also for revert?
What is the merge base for a revert? In What are the three files in a 3-way merge for interactive rebasing using git and meld? it is quite clear, but its hard to imagine this for a revert.
A - B - C - D - C^-1
(If I want to revert C at the end.)
Yes, there is a base. (Side note: this code has changed a lot since I looked at it years ago. I picked up some of this for my recent cherry-pick answer, which you have linked here.)
Both git cherry-pick and git revert are implemented by the same source files (builtin/revert.c and sequencer.c).
As you say, the tricky part is deciding what to fake up for the merge base. In your example, we're undoing the B-to-C diffs. Here's the actual source code (in sequencer.c), stripped down somewhat:
if (opts->action == REPLAY_REVERT) {
base = commit;
base_label = msg.label;
next = parent;
next_label = msg.parent_label;
strbuf_addstr(&msgbuf, "Revert \"");
strbuf_addstr(&msgbuf, msg.subject);
strbuf_addstr(&msgbuf, "\"\n\nThis reverts commit ");
strbuf_addstr(&msgbuf, oid_to_hex(&commit->object.oid));
if (commit->parents && commit->parents->next) {
strbuf_addstr(&msgbuf, ", reversing\nchanges made to ");
strbuf_addstr(&msgbuf, oid_to_hex(&parent->object.oid));
}
strbuf_addstr(&msgbuf, ".\n");
} else {
[this is the cherry-pick case, included just for completeness]
const char *p;
base = parent;
base_label = msg.parent_label;
next = commit;
next_label = msg.label;
When we enter here, commit points to data for C and parent points to data for B. The assignment to variable base is what sets the merge base, and next-vs-base is what to bring in. For cherry-pick, the commit's parent (possibly chosen via -m) is the merge base. For revert, the commit itself is the merge base and the parent (again possibly from -m) is what-to-bring-in.
The other way to get the same effect (which is how this was done many years ago, and until recently, I thought this was still being used) is to reverse-apply a commit as produced by git format-patch. In this case, the constructed base version is the second hash (the B part from the A..B part of a textual diff):
/*
* This represents a "patch" to a file, both metainfo changes
* such as creation/deletion, filemode and content changes represented
* as a series of fragments.
*/
struct patch {
[snip]
char old_sha1_prefix[41];
char new_sha1_prefix[41];
static void reverse_patches(struct patch *p)
{
[snip]
swap(p->old_sha1_prefix, p->new_sha1_prefix);
The reverse_patches function is called after extracting the text into a series of patches, i.e., after the code that extracts the hashes from the index lines, putting the A and B parts into the old and new prefix fields. Then (after reverse_patches), when actually applying each patch, git uses the saved old and new sha1 values to fake a 3-way merge (if git am is given --3way). So by reverse-applying a text patch, we would get the new file as the base and the original as the target, just as with the sequencer.c code.
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