The merge API helps a program to reconcile two competing sets of improvements to some files (e.g., unregistered changes from the work tree versus changes involved in switching to a new branch), reporting conflicts if found. The library called through this API is responsible for a few things.
determining which trees to merge (recursive ancestor consolidation);
lining up corresponding files in the trees to be merged (rename detection, subtree shifting), reporting edge cases like add/add and rename/rename conflicts to the user;
performing a three-way merge of corresponding files, taking path-specific merge drivers (specified in .gitattributes) into account.
Perform a three-way single-file merge in core. This is a thin wrapper around xdl_merge that takes the path and any merge backend specified in .gitattributes or .git/info/attributes into account. Returns 0 for a clean merge.
allocates an mmbuffer_t variable for the result;
allocates and fills variables with the file’s original content and two modified versions (using read_mmfile, for example);
reads the output from result_buf.ptr and result_buf.size;
releases buffers when finished (free(ancestor.ptr); free(ours.ptr); free(theirs.ptr); free(result_buf.ptr);).
If the modifications do not merge cleanly, ll_merge will return a nonzero value and result_buf will generally include a description of the conflict bracketed by markers such as the traditional <<<<<<< and >>>>>>>.
The ancestor_label, our_label, and their_label parameters are used to label the different sides of a conflict if the merge driver supports this.
The flag parameter is a bitfield:
The LL_OPT_VIRTUAL_ANCESTOR bit indicates whether this is an internal merge to consolidate ancestors for a recursive merge.
The LL_OPT_FAVOR_MASK bits allow local conflicts to be automatically resolved in favor of one side or the other (as in git merge-file --ours/--theirs/--union). They can be populated by create_ll_flag, whose argument can be XDL_MERGE_FAVOR_OURS, XDL_MERGE_FAVOR_THEIRS, or XDL_MERGE_FAVOR_UNION.
Talk about <merge-recursive.h> and merge_file():
merge_trees() to merge with rename detection
merge_recursive() for ancestor consolidation
try_merge_command() for other strategies
(Daniel, Miklos, Stephan, JC)