Commit testing fails after nightly testing when there is no nightly at the begin date defined during nightly tests

[chrissimpkins]

After identifying a rustc regression in a March 2019 nightly with a shell script that executes rustc directly, the tool fails and exits with a non-zero exit status code during the attempt to bisect by commits with the following error:

checking nightly-2019-01-01
std for x86_64-unknown-linux-gnu: 56.94 MB / 56.94 MB [==========================================] 100.00 % 4.74 MB/s uninstalling nightly-2019-01-01
verifying the start of the range does not reproduce the regression
std for x86_64-unknown-linux-gnu: 56.94 MB / 56.94 MB [==========================================] 100.00 % 2.96 MB/s uninstalling nightly-2019-01-01
tested nightly-2019-01-01, got No
confirmed the start of the range does not reproduce the regression
verifying the end of the range reproduces the regression
std for x86_64-unknown-linux-gnu: 17.53 MB / 17.53 MB [==========================================] 100.00 % 4.07 MB/s uninstalling nightly-2020-02-01
tested nightly-2020-02-01, got Yes
confirmed the end of the range reproduces the regression
std for x86_64-unknown-linux-gnu: 170.27 MB / 170.27 MB [========================================] 100.00 % 4.98 MB/s uninstalling nightly-2019-07-18
tested nightly-2019-07-18, got Yes
uninstalling nightly-2019-04-10
failed to install nightly-2019-04-10: NotFound { url: "https://static.rust-lang.org/dist/2019-04-10/rustc-nightly-x86_64-unknown-linux-gnu.tar.gz", spec: Nightly { date: 2019-04-10Z } }
std for x86_64-unknown-linux-gnu: 59.01 MB / 59.01 MB [==========================================] 100.00 % 4.16 MB/s uninstalling nightly-2019-04-09
tested nightly-2019-04-09, got Yes
std for x86_64-unknown-linux-gnu: 59.27 MB / 59.27 MB [==========================================] 100.00 % 9.24 MB/s uninstalling nightly-2019-04-11
tested nightly-2019-04-11, got Yes
std for x86_64-unknown-linux-gnu: 55.88 MB / 55.88 MB [==========================================] 100.00 % 7.68 MB/s uninstalling nightly-2019-02-19
tested nightly-2019-02-19, got No
std for x86_64-unknown-linux-gnu: 57.67 MB / 57.67 MB [=========================================] 100.00 % 11.12 MB/s uninstalling nightly-2019-03-15
tested nightly-2019-03-15, got Yes
std for x86_64-unknown-linux-gnu: 56.39 MB / 56.39 MB [==========================================] 100.00 % 4.80 MB/s uninstalling nightly-2019-03-03
tested nightly-2019-03-03, got No
std for x86_64-unknown-linux-gnu: 58.08 MB / 58.08 MB [==========================================] 100.00 % 5.39 MB/s uninstalling nightly-2019-03-09
tested nightly-2019-03-09, got Yes
std for x86_64-unknown-linux-gnu: 56.62 MB / 56.62 MB [==========================================] 100.00 % 4.92 MB/s uninstalling nightly-2019-03-06
tested nightly-2019-03-06, got No
uninstalling nightly-2019-03-07
failed to install nightly-2019-03-07: NotFound { url: "https://static.rust-lang.org/dist/2019-03-07/rustc-nightly-x86_64-unknown-linux-gnu.tar.gz", spec: Nightly { date: 2019-03-07Z } }
std for x86_64-unknown-linux-gnu: 56.47 MB / 56.47 MB [==========================================] 100.00 % 2.73 MB/s uninstalling nightly-2019-03-08
tested nightly-2019-03-08, got Yes
searched toolchains nightly-2019-01-01 through nightly-2020-02-01
regression in nightly-2019-03-08
fetching https://static.rust-lang.org/dist/2019-03-08/channel-rust-nightly-git-commit-hash.txt
nightly manifest 2019-03-08: 40 B / 40 B [=====================================================] 100.00 % 489.60 KB/s converted 2019-03-08 to 88f755f8a84df1d9e6b17cf10c96ae8b93481b2e
fetching https://static.rust-lang.org/dist/2019-03-07/channel-rust-nightly-git-commit-hash.txt
Tarball not found at https://static.rust-lang.org/dist/2019-03-07/channel-rust-nightly-git-commit-hash.txt

When I enter the URL manually, there is a git commit hash at https://static.rust-lang.org/dist/2019-03-06/channel-rust-nightly-git-commit-hash.txt and https://static.rust-lang.org/dist/2019-03-08/channel-rust-nightly-git-commit-hash.txt, but not at https://static.rust-lang.org/dist/2019-03-07/channel-rust-nightly-git-commit-hash.txt.

It looks like we need to maintain information about missing nightlies during the nightly test phase so that we can step back to the next most recent nightly for the commit test range definition.