main: optimise "runtime" steps (#80)

gobin maintains binaries in its own cache. The key for a cache entry is
a function of main package path, version and build tags. Assuming nobody
tampers with the build cache (making it readonly is the subject of #5),
then we need only install to the cache entry if a file does not already
exist.

Also, optimise go list calls by using -find; we don't care for the
dependencies in these steps.

No change in tests because there should be no change in behaviour.

With some incredibly unscientific timing tests on my machine, this
reduces the time taken for "gobin -run github.com/myitcv/gobin -help"
from 0.636s to 0.246s. "gobin -help" times at 0.070s which means ~0.176s
of gobin "overhead".

.travis.yml

@@ -5,10 +5,14 @@ cache:
   - $GOPATH/pkg/mod/cache
 
 go:
-  - "1.11.12"
-  - "1.12.7"
+  - "1.11.13"
+  - "1.12.9"
   - "1.13beta1"
 
+branches:
+  only:
+    - master
+
 os:
   - linux
   - osx

_scripts/known_diffs/go1.11.12/badly_sorted_vendor_modules.txt.diff → _scripts/known_diffs/go1.11.13/badly_sorted_vendor_modules.txt.diff

@@ -1,9 +1,9 @@
 diff --git b/vendor/modules.txt a/vendor/modules.txt
-index 49894ad..5b8d57c 100644
+index 01abe41..ebc5500 100644
 --- b/vendor/modules.txt
 +++ a/vendor/modules.txt
 @@ -1,16 +1,16 @@
- # github.com/rogpeppe/go-internal v1.3.0
+ # github.com/rogpeppe/go-internal v1.3.1
  github.com/rogpeppe/go-internal/cmd/txtar-addmod
 -github.com/rogpeppe/go-internal/module
 -github.com/rogpeppe/go-internal/txtar

_scripts/known_diffs/go1.12.7/badly_sorted_vendor_modules.txt.diff → _scripts/known_diffs/go1.12.9/badly_sorted_vendor_modules.txt.diff

@@ -1,9 +1,9 @@
 diff --git b/vendor/modules.txt a/vendor/modules.txt
-index 49894ad..5b8d57c 100644
+index 01abe41..ebc5500 100644
 --- b/vendor/modules.txt
 +++ a/vendor/modules.txt
 @@ -1,16 +1,16 @@
- # github.com/rogpeppe/go-internal v1.3.0
+ # github.com/rogpeppe/go-internal v1.3.1
  github.com/rogpeppe/go-internal/cmd/txtar-addmod
 -github.com/rogpeppe/go-internal/module
 -github.com/rogpeppe/go-internal/txtar

go.mod

@@ -1,5 +1,5 @@
 module github.com/myitcv/gobin
 
-require github.com/rogpeppe/go-internal v1.3.0
+require github.com/rogpeppe/go-internal v1.3.1
 
 go 1.11

go.sum

@@ -3,8 +3,8 @@ github.com/kr/pretty v0.1.0/go.mod h1:dAy3ld7l9f0ibDNOQOHHMYYIIbhfbHSm3C4ZsoJORN
 github.com/kr/pty v1.1.1/go.mod h1:pFQYn66WHrOpPYNljwOMqo10TkYh1fy3cYio2l3bCsQ=
 github.com/kr/text v0.1.0 h1:45sCR5RtlFHMR4UwH9sdQ5TC8v0qDQCHnXt+kaKSTVE=
 github.com/kr/text v0.1.0/go.mod h1:4Jbv+DJW3UT/LiOwJeYQe1efqtUx/iVham/4vfdArNI=
-github.com/rogpeppe/go-internal v1.3.0 h1:RR9dF3JtopPvtkroDZuVD7qquD0bnHlKSqaQhgwt8yk=
-github.com/rogpeppe/go-internal v1.3.0/go.mod h1:M8bDsm7K2OlrFYOpmOWEs/qY81heoFRclV5y23lUDJ4=
+github.com/rogpeppe/go-internal v1.3.1 h1:pgz0lCb+F99TrCwoy7d83j5kI//45fBQ34KzZ7t5as0=
+github.com/rogpeppe/go-internal v1.3.1/go.mod h1:xXDCJY+GAPziupqXw64V24skbSoqbTEfhy4qGm1nDQc=
 gopkg.in/check.v1 v1.0.0-20180628173108-788fd7840127 h1:qIbj1fsPNlZgppZ+VLlY7N33q108Sa+fhmuc+sWQYwY=
 gopkg.in/check.v1 v1.0.0-20180628173108-788fd7840127/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/errgo.v2 v2.1.0 h1:0vLT13EuvQ0hNvakwLuFZ/jYrLp5F3kcWHXdRggjCE8=

main.go

@@ -364,25 +364,30 @@ func mainerr() error {
 			base := filepath.Base(mp.Target)
 			target := filepath.Join(gobin, base)
 
-			// optimistically remove our target in case we are installing over self
-			// TODO work out what to do for Windows
-			if mp.ImportPath == "github.com/myitcv/gobin" {
-				_ = os.Remove(target)
-			}
-
-			var stdout bytes.Buffer
-
-			installCmd := goCommand("install")
-			if *fTags != "" {
-				installCmd.Args = append(installCmd.Args, "-tags", *fTags)
-			}
-			installCmd.Args = append(installCmd.Args, mp.ImportPath)
-			installCmd.Dir = pkg.wd
-			installCmd.Env = append(buildEnv(localCacheProxy), "GOBIN="+gobin)
-			installCmd.Stdout = &stdout
-
-			if err := installCmd.run(); err != nil {
-				return err
+			// Only install if the target (within the gobin cache) does not
+			// exist. For now this cache is not read-only so this isn't as
+			// safe as it could/should be, but people shouldn't be messing with
+			// the cache anyway. The target in the cache is already hash by
+			// build tags so we should never have "overlapping" gobin runs.
+			if _, err := os.Stat(target); err != nil {
+				if !os.IsNotExist(err) {
+					return fmt.Errorf("failed to read %v: %v", target, err)
+				}
+				// optimistically remove our target in case we are installing over self
+				// TODO work out what to do for Windows
+				if mp.ImportPath == "github.com/myitcv/gobin" {
+					_ = os.Remove(target)
+				}
+				installCmd := goCommand("install")
+				if *fTags != "" {
+					installCmd.Args = append(installCmd.Args, "-tags", *fTags)
+				}
+				installCmd.Args = append(installCmd.Args, mp.ImportPath)
+				installCmd.Dir = pkg.wd
+				installCmd.Env = append(buildEnv(localCacheProxy), "GOBIN="+gobin)
+				if err := installCmd.run(); err != nil {
+					return err
+				}
 			}
 
 			switch {
@@ -511,7 +516,7 @@ func (a *arg) list(proxy string) error {
 
 	var stdout bytes.Buffer
 
-	listCmd := goCommand("list", "-json", a.pkgPatt)
+	listCmd := goCommand("list", "-find", "-json", a.pkgPatt)
 	listCmd.Dir = a.wd
 	listCmd.Stdout = &stdout
 	listCmd.Env = env
@@ -649,7 +654,7 @@ func (a *arg) list(proxy string) error {
 			// target module (including replace directives), list to ensure they
 			// have been resolved
 
-			listCmd := goCommand("list", "-json", pkg.ImportPath)
+			listCmd := goCommand("list", "-find", "-json", pkg.ImportPath)
 			listCmd.Dir = a.wd
 			listCmd.Env = buildEnv(proxy)