debugdeltachain: protect against 0 readsize If you don't read any data, read density should be 1.

debugdeltachain: avoid division by zero when a chain is empty The two ratios chainratio and extraratio are computed using dividers that may be zero when the file is empty. As the denominators are integers, the limit of the ratio "just before zero" is the numerator value itself. If the numerator itself is zero, the ratio value is still meaningful: in both cases, a "good" value is a low ratio, and a size of zero is the optimal case.

tests: prevent conflict markers in test case from triggering warnings Broadly, having lines that begin with several '<' tends to trigger tools that try to warn you about unresolved conflicts. It will save some headaches to tweak this string literal to avoid that. Differential Revision: https://phab.mercurial-scm.org/D3897

revlog: enforce chunk slicing down to a certain size Limit maximum chunk size to 4x final size when reading a revision from a revlog. We only apply this logic when the target size is known from the revlog. Ideally, revlog's delta chain would be written in a way that does not trigger this extra slicing often. However, having this second guarantee that we won't read unexpectedly large amounts of memory in all cases is important for the future. Future delta chain building algorithms might have good reason to create delta chain with such characteristics. Including this code in core as soon as possible will make Mercurial 4.7 forward-compatible with such improvement.

revlog: postprocess chunk to slice them down to a certain size After the density slicing is done, we enforce a maximum chunk size to avoid memory consumption issue.

revlog: add function to slice chunk down to a given size It is possible to encounter situations where the slicing based on density did not achieve chunk smaller than the 4*textlength limit. To avoid extra memory consumption in those cases, we need to be able to break down chunk to a given size. Actual caller comes in the next changesets.

revlog: extract density based slicing into its own function We are going to introduce another slicing step. We start by extracting the existing one into its own function.

revlog: document and test _slicechunk

revlog: add a doctest for _segmentspan

revlog: add a doctest to _trimchunk