On Thu, Aug 27, 2020 at 09:15:56AM -0400, Derrick Stolee wrote: > On 8/27/2020 2:39 AM, Abhishek Kumar wrote: > > Thinking about this, I feel creating a new section called "Handling > > Mixed Generation Number Chains" made more sense: > > > > ## Handling Mixed Generation Number Chains > > > > With the introduction of generation number v2 and generation data chunk, > > the following scenario is possible: > > > > 1. "New" Git writes a commit-graph with a GDAT chunk. > > 2. "Old" Git writes a split commit-graph on top without a GDAT chunk. > > I like the idea of this section, and this setup is good. > > > The commits in the lower layer will be interpreted as having very large > > generation values (commit date plus offset) compared to the generation > > numbers in the top layer (toplogical level). This violates the > > expectation that the generation of a parent is strictly smaller than the > > generation of a child. In such cases, we revert to using topological > > levels for all layers to maintain backwards compatability. > > s/toplogical/topological > > But also, we don't want to phrase this as "in this case, we do the wrong > thing" but instead > > A naive approach of using the newest available generation number from > each layer would lead to violated expectations: the lower layer would > use corrected commit dates which are much larger than the topological > levels of the higher layer. For this reason, Git inspects each layer > to see if any layer is missing corrected commit dates. In such a case, > Git only uses topological levels. > > > When writing a new layer in split commit-graph, we write a GDAT chunk > > only if the topmost layer has a GDAT chunk. This guarantees that if a > > lyer has GDAT chunk, all lower layers must have a GDAT chunk as well. > > s/lyer/layer > > Perhaps leaving this at a higher level than referencing "GDAT chunk" is > advisable. Perhaps use "we write corrected commit dates" or "all lower > layers must store corrected commit dates as well", for example. > > > Rewriting layers follows similar approach: if the topmost layer below > > set of layers being rewriteen (in the split commit-graph chain) exists, > > and it does not contain GDAT chunk, then the result of rewrite does not > > have GDAT chunks either. > > This could use more positive language to make it clear that sometimes > we _do_ want to write corrected commit dates when merging layers: > > When merging layers, we do not consider whether the merged layers had > corrected commit dates. Instead, the new layer will have corrected > commit dates if and only if all existing layers below the new layer > have corrected commit dates. Thanks, that is a great suggestion! Using positive language is more straightforward and easier to understand. > > Thanks, > -Stolee
