Commits on Nov 10, 2021

1. builders/cover: register coverage without changing line numbers …

   The coverage support in the builder works in the following way:
   
   1. Run `go tool cover` to create an instrumented source file, for example:
   
       ```
       $ go test -work -cover database/sql
       WORK=/var/folders/yf/4v4h0_t53h30t_8vc90d1gx40000gp/T/go-build105184680/
   
       $ find $WORK -name *cover.go
       ...
   
       $ less /var/folders/yf/4v4h0_t53h30t_8vc90d1gx40000gp/T/go-build105184680//b059/sql.cover.go
       ...
       func Register(name string, driver driver.Driver) {GoCover_2_343132323961636265643234.Count[0] = 1;
               driversMu.Lock()
               defer driversMu.Unlock()
               if driver == nil {GoCover_2_343132323961636265643234.Count[3] = 1;
                       panic("sql: Register driver is nil")
               }
               GoCover_2_343132323961636265643234.Count[1] = 1;if _, dup := drivers[name]; dup {GoCover_2_343132323961636265643234.Count[4] = 1;
                       panic("sql: Register called twice for driver " + name)
               }
               GoCover_2_343132323961636265643234.Count[2] = 1;drivers[name] = driver
       }
   
       ...
       // appended to end of file
       var GoCover_2_343132323961636265643234 = struct {
               Count     [727]uint32
               Pos       [3 * 727]uint32
               NumStmt   [727]uint16
       } {
       ...
       ```
   
   2. Register each file's coverage (above, `GoCover_2_343132323961636265643234`) with
      [coverdata](https://pkg.go.dev/github.com/bazelbuild/rules_go@v0.29.0/go/tools/coverdata).
      This requires adding or updating an import for that package and calling `RegisterFile` in `init()`.
      This is done by using `go/parser` and `go/ast` to modify the AST and `go/format` to print it
      back out to the instrumented source file.
   
   The problem is that this changes line numbers, resulting in panics
   that no longer identify the correct stack trace.
   
   `go tool cover` avoids this issue because it places the coverage
   variable increments on the same line as control flow branch of
   interest to avoid this problem, which it accomplishes by using the
   [cmd/internal/edit](https://pkg.go.dev/cmd/internal/edit@go1.17.3)
   package.
   
   This change brings that approach to rules_go coverage instrumentation:
   
   1. Copy in the cmd/internal/edit package for bytebuffer-based
      editing.
   
   2. Update the `registerCoverage` step to use that instead of AST
      modifications & `go/format`.
   
   Fixes #2990

   

   robfig committed Nov 10, 2021
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