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diff --git a/Documentation/user-manual.txt b/Documentation/user-manual.txt index c7fdf25e27..e399685d6c 100644 --- a/Documentation/user-manual.txt +++ b/Documentation/user-manual.txt @@ -2554,6 +2554,72 @@ branches into their own work. For true distributed development that supports proper merging, published branches should never be rewritten. +[[bisect-merges]] +Why bisecting merge commits can be harder than bisecting linear history +----------------------------------------------------------------------- + +The gitlink:git-bisect[1] command correctly handles history that +includes merge commits. However, when the commit that it finds is a +merge commit, the user may need to work harder than usual to figure out +why that commit introduced a problem. + +Imagine this history: + +................................................ + ---Z---o---X---...---o---A---C---D + \ / + o---o---Y---...---o---B +................................................ + +Suppose that on the upper line of development, the meaning of one +of the functions that exists at Z is changed at commit X. The +commits from Z leading to A change both the function's +implementation and all calling sites that exist at Z, as well +as new calling sites they add, to be consistent. There is no +bug at A. + +Suppose that in the meantime on the lower line of development somebody +adds a new calling site for that function at commit Y. The +commits from Z leading to B all assume the old semantics of that +function and the callers and the callee are consistent with each +other. There is no bug at B, either. + +Suppose further that the two development lines merge cleanly at C, +so no conflict resolution is required. + +Nevertheless, the code at C is broken, because the callers added +on the lower line of development have not been converted to the new +semantics introduced on the upper line of development. So if all +you know is that D is bad, that Z is good, and that +gitlink:git-bisect[1] identifies C as the culprit, how will you +figure out that the problem is due to this change in semantics? + +When the result of a git-bisect is a non-merge commit, you should +normally be able to discover the problem by examining just that commit. +Developers can make this easy by breaking their changes into small +self-contained commits. That won't help in the case above, however, +because the problem isn't obvious from examination of any single +commit; instead, a global view of the development is required. To +make matters worse, the change in semantics in the problematic +function may be just one small part of the changes in the upper +line of development. + +On the other hand, if instead of merging at C you had rebased the +history between Z to B on top of A, you would have gotten this +linear history: + +................................................................ + ---Z---o---X--...---o---A---o---o---Y*--...---o---B*--D* +................................................................ + +Bisecting between Z and D* would hit a single culprit commit Y*, +and understanding why Y* was broken would probably be easier. + +Partly for this reason, many experienced git users, even when +working on an otherwise merge-heavy project, keep the history +linear by rebasing against the latest upstream version before +publishing. + [[advanced-branch-management]] Advanced branch management ========================== |