Stop assuming there will always be a git commit

[teor2345]

Motivation

Since #1741, Zebra assumes that there will always be a .git directory. Even before that, Zebra assumed that there would always be some kind of git commit hash available.

Solution

Enable builds where:

• there is no google cloud git commit env var, and
• there is no .git directory.

By making all vergen env vars optional, and skipping any env vars that don't exist.

The code in this pull request has:

• Documentation Comments
• Acceptance Tests

Manual Test Output

Cause a port/database conflict panic:

zebrad start > /dev/null &
zebrad start
killall zebrad

Without .git directory (and no google cloud git commit env var):

Metadata:                                                                                                                                                                                      
version: 1.0.0-alpha.6                                                                                                                                                                         Zcash network: Mainnet                                                                                                                                                                                                                                                                                                                                                                          ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ SPANTRACE ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━                                                                                                                                                                                                                                                                                                            
   0: zebrad::application:: with net="Main"                                                                                                                                                    
      at zebrad/src/application.rs:292   

Note that the target triple is also skipped, even though we can discover it directly from cargo. See rustyhorde/vergen#64.

With a .git directory:

Metadata:                                
version: 1.0.0-alpha.6
branch: vergen-no-git-fix
git commit: 102e465                                                                                                                                                                            
commit timestamp: 2021-04-20T07:05:45+00:00
target triple: x86_64-unknown-linux-gnu                                                        
Zcash network: Mainnet

  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ SPANTRACE ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
                                                                                                                                                                                               
   0: zebrad::application:: with zebrad="102e465" net="Main"                                                                                                                                   
      at zebrad/src/application.rs:290    

Review

This fix is urgent because the CD task is failing.

Related Issues

Caused by PR #2029.

Follow Up Work

Fix the cargo target triple and supply a better git commit version.