Inifinite loop(?) in core::resolver

[SimonSapin]

When building https://github.com/ecoal95/rust-offscreen-rendering-context, Cargo seems to get in an infinite loop after printing "Updating git repository https://github.com/servo/libfontconfig"

Note: it is possible that my $CARGO_HOME is in a broken state which triggers the issue, and that you might not reproduce it easily.

Sample output from strace:

madvise(0x7f4efa643000, 376832, MADV_DONTNEED) = 0
madvise(0x7f4efa4b2000, 4096, MADV_DONTNEED) = 0
madvise(0x7f4efa212000, 4096, MADV_DONTNEED) = 0
madvise(0x7f4efa520000, 4096, MADV_DONTNEED) = 0
madvise(0x7f4efa166000, 684032, MADV_DONTNEED) = 0
madvise(0x7f4efa643000, 376832, MADV_DONTNEED) = 0
madvise(0x7f4efa212000, 4096, MADV_DONTNEED) = 0
madvise(0x7f4efa103000, 4096, MADV_DONTNEED) = 0
madvise(0x7f4efa619000, 28672, MADV_DONTNEED) = 0
madvise(0x7f4efa166000, 684032, MADV_DONTNEED) = 0
madvise(0x7f4efa69a000, 20480, MADV_DONTNEED) = 0
madvise(0x7f4efa643000, 352256, MADV_DONTNEED) = 0
madvise(0x7f4efa212000, 4096, MADV_DONTNEED) = 0
madvise(0x7f4efa166000, 684032, MADV_DONTNEED) = 0
madvise(0x7f4efa510000, 32768, MADV_DONTNEED) = 0
madvise(0x7f4efa6c4000, 360448, MADV_DONTNEED) = 0
madvise(0x7f4efa212000, 4096, MADV_DONTNEED) = 0
madvise(0x7f4efa103000, 4096, MADV_DONTNEED) = 0
madvise(0x7f4efa166000, 684032, MADV_DONTNEED) = 0
madvise(0x7f4efa619000, 28672, MADV_DONTNEED) = 0
madvise(0x7f4efa6c4000, 360448, MADV_DONTNEED) = 0
madvise(0x7f4efa212000, 4096, MADV_DONTNEED) = 0
madvise(0x7f4efa721000, 4096, MADV_DONTNEED) = 0
madvise(0x7f4efa166000, 61440, MADV_DONTNEED) = 0
madvise(0x7f4efa176000, 618496, MADV_DONTNEED) = 0
madvise(0x7f4efa6c4000, 360448, MADV_DONTNEED) = 0
madvise(0x7f4efa212000, 4096, MADV_DONTNEED) = 0
madvise(0x7f4efa176000, 618496, MADV_DONTNEED) = 0
madvise(0x7f4efa6c4000, 360448, MADV_DONTNEED) = 0
madvise(0x7f4efa212000, 4096, MADV_DONTNEED) = 0
madvise(0x7f4efa176000, 618496, MADV_DONTNEED) = 0
madvise(0x7f4efa6c4000, 360448, MADV_DONTNEED) = 0
getrandom(" \256\225Y\344\375\25c\2121r\320\331^\31\250R$\367\364|A\f(l\356{\0t1\257\357"..., 2048, 0) = 2048
madvise(0x7f4efa212000, 4096, MADV_DONTNEED) = 0
madvise(0x7f4efa61a000, 4096, MADV_DONTNEED) = 0
madvise(0x7f4efa18c000, 528384, MADV_DONTNEED) = 0
madvise(0x7f4efa176000, 86016, MADV_DONTNEED) = 0
madvise(0x7f4efa6c4000, 360448, MADV_DONTNEED) = 0
madvise(0x7f4efa630000, 4096, MADV_DONTNEED) = 0
madvise(0x7f4efa72a000, 28672, MADV_DONTNEED) = 0
madvise(0x7f4efa18c000, 503808, MADV_DONTNEED) = 0
madvise(0x7f4efa643000, 376832, MADV_DONTNEED) = 0
madvise(0x7f4efa18c000, 503808, MADV_DONTNEED) = 0
madvise(0x7f4efa6c4000, 385024, MADV_DONTNEED) = 0
madvise(0x7f4efa643000, 376832, MADV_DONTNEED) = 0

Sample stack from gdb:

#0  0x0000559fcb0565d6 in collections::hash::set::_$LT$impl$GT$::clone::clone::h16694492552681060282 ()
#1  0x0000559fcb055bec in core::resolver::_$LT$impl$GT$::clone::h0ae172b591c18f5b3Ze ()
#2  0x0000559fcb05b3c4 in core::resolver::_$LT$impl$GT$::clone::h09850ffffecf5d85X9e ()
#3  0x0000559fcb05d72d in core::resolver::activate_deps_loop::h36a601ba1ee5248cPif ()
#4  0x0000559fcb05c0ca in core::resolver::resolve::h056102269dfa3413xaf ()
#5  0x0000559fcb094e2a in ops::resolve::resolve_with_previous::h5e01604dc4e96231l4p ()
#6  0x0000559fcb0938c8 in ops::resolve::resolve_pkg::h0b5095fee43d8d02V2p ()
#7  0x0000559fcb092907 in ops::cargo_compile::resolve_dependencies::hbc91c43ac45df08csEh ()
#8  0x0000559fcb08dc8a in ops::cargo_compile::compile_pkg::h2e9bdb08763ca8e6uHh ()
#9  0x0000559fcb08c7c2 in ops::cargo_compile::compile::hb0f62d5be04d643eoBh ()
#10 0x0000559fcb1847be in ops::cargo_test::compile_tests::h88f6d000da6b812fr3o ()
#11 0x0000559fcb1808aa in ops::cargo_test::run_tests::h2152735aee659366sZo ()
#12 0x0000559fcb0029bd in call_main_without_stdin::call_main_without_stdin::h9555677425742150942 ()
#13 0x0000559fcaf9b26a in execute::hf7cebfaab2e8ad5b5ca ()
#14 0x0000559fcaf93777 in call_main_without_stdin::call_main_without_stdin::h8017624274300204301 ()
#15 0x0000559fcaf909d3 in main::h3d3ac0797c5e9e66Tca ()
#16 0x0000559fcb556bd5 in sys_common::unwind::try::try_fn::h8623127493126415308 ()
#17 0x0000559fcb5513b9 in __rust_try ()
#18 0x0000559fcb556870 in rt::lang_start::h426b3aba4736785fsbx ()
#19 0x00007f4f021d7610 in __libc_start_main () from /usr/lib/libc.so.6
#20 0x0000559fcaf902c5 in _start ()

[alexcrichton]

This is unlikely to be an infinite loop but rather just a pathological case in resolve. Sometimes if constraints are too strict the resolver will spend basically all day backtracking.

I think this is a dupe of #2090 so I'm going to close this in favor of that.