bundle2: avoid unbound read when seeking Currently, seekableunbundlepart.seek() will perform a read() during seek operations. This will allocate a buffer to hold the raw data over the seek distance. This can lead to very large allocations and cause performance to suffer. We change the code to perform read(32768) in a loop to avoid potentially large allocations. `hg perfbundleread` on an uncompressed Firefox bundle reveals a performance impact: ! bundle2 iterparts() ! wall 2.992605 comb 2.990000 user 2.260000 sys 0.730000 (best of 4) ! bundle2 iterparts() seekable ! wall 3.863810 comb 3.860000 user 3.000000 sys 0.860000 (best of 3) ! bundle2 part seek() ! wall 6.213387 comb 6.200000 user 3.350000 sys 2.850000 (best of 3) ! wall 3.820347 comb 3.810000 user 2.980000 sys 0.830000 (best of 3) Since seekable bundle parts are (only) used by bundlerepo, this /may/ speed up initial loading of bundle-based repos. But any improvement will likely only be noticed on very large bundles. Differential Revision: https://phab.mercurial-scm.org/D1394