From b6c5f5f40635ed83c0d0439fe145ed5b96cab3c5 Mon Sep 17 00:00:00 2001
From: Paul Duncan <pabs@pablotron.org>
Date: Tue, 22 Feb 2022 21:57:44 -0500
Subject: add atomictemp

---
 atomictemp/atomictemp.go      | 48 ++++++++++++++++++++++++++++++
 atomictemp/atomictemp_test.go | 68 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 116 insertions(+)
 create mode 100644 atomictemp/atomictemp.go
 create mode 100644 atomictemp/atomictemp_test.go

diff --git a/atomictemp/atomictemp.go b/atomictemp/atomictemp.go
new file mode 100644
index 0000000..b56d095
--- /dev/null
+++ b/atomictemp/atomictemp.go
@@ -0,0 +1,48 @@
+// Create a temporary file in the directory of the given destination
+// file, then rename the temporary file over the destination file if
+// writes to the temporary file complete without error.
+//
+// Used to atomically update files.
+//
+// Note: This is not guaranteed and depends on the sync semantics of the
+// underlying filesystem.
+package atomictemp
+
+import (
+  "io"
+  "os"
+  "path/filepath"
+)
+
+// Create a temporary file in the directory of the given destination
+// file, then rename the temporary file over the destination file if
+// writes to the temporary file complete without error.
+//
+// Used to atomically update files.
+//
+// Note: This is not guaranteed and depends on the sync semantics of the
+// underlying filesystem.
+func Create(dstPath string, fn func(io.Writer) error) error {
+  // open temp output file
+  f, err := os.CreateTemp(filepath.Dir(dstPath), "")
+  if err != nil {
+    return err
+  }
+
+  // build absolute path to temporary file
+  // tmpPath := filepath.Join(filepath.Dir(dstPath), f.Name())
+  defer os.Remove(f.Name())
+
+  // invoke callback
+  if err = fn(f); err != nil {
+    return err
+  }
+
+  // close temp file
+  if err = f.Close(); err != nil {
+    return err
+  }
+
+  // rename to destination file, return result
+  return os.Rename(f.Name(), dstPath)
+}
diff --git a/atomictemp/atomictemp_test.go b/atomictemp/atomictemp_test.go
new file mode 100644
index 0000000..401c730
--- /dev/null
+++ b/atomictemp/atomictemp_test.go
@@ -0,0 +1,68 @@
+package atomictemp
+
+import (
+  "errors"
+  "io"
+  "os"
+  "path/filepath"
+  "testing"
+)
+
+func TestCreate(t *testing.T) {
+  dir, err := os.MkdirTemp("", "")
+  if err != nil {
+    t.Error(err)
+  }
+
+  // test good file
+  goodTests := []string { "foo", "bar", "baz" }
+  for _, test := range(goodTests) {
+    t.Run(test, func(t *testing.T) {
+      // build destination path
+      path := filepath.Join(dir, test)
+
+      // create temp file
+      err := Create(path, func(f io.Writer) error {
+        _, err := f.Write([]byte(test))
+        return err
+      })
+
+      if err != nil {
+        t.Error(err)
+        return
+      }
+
+      if got, err := os.ReadFile(path); err != nil {
+        t.Error(err)
+      } else if string(got) != test {
+        t.Errorf("got \"%s\", exp \"%s\"", string(got), test)
+      }
+    })
+  }
+
+  t.Run("badDir", func(t *testing.T) {
+    // build nonsense path to destination file
+    badPath := filepath.Join(dir, "does/not/exist")
+
+    err := Create(badPath, func(_ io.Writer) error {
+      return nil
+    })
+
+    if err == nil {
+      t.Errorf("got success, exp error")
+    }
+  })
+
+  t.Run("badFunc", func(t *testing.T) {
+    // build path
+    path := filepath.Join(dir, "badFunc")
+
+    err := Create(path, func(_ io.Writer) error {
+      return errors.New("ack!")
+    })
+
+    if err == nil {
+      t.Errorf("got success, exp error")
+    }
+  })
+}
-- 
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