feat: return better error if file size is too big to upload (#7775)

* feat: return better error if file size is too big to upload
* Use a limit writer to capture actual tar size

coderd/util/xio/limitwriter.go

@@ -0,0 +1,43 @@
+package xio
+
+import (
+	"io"
+
+	"golang.org/x/xerrors"
+)
+
+var ErrLimitReached = xerrors.Errorf("i/o limit reached")
+
+// LimitWriter will only write bytes to the underlying writer until the limit is reached.
+type LimitWriter struct {
+	Limit int64
+	N     int64
+	W     io.Writer
+}
+
+func NewLimitWriter(w io.Writer, n int64) *LimitWriter {
+	// If anyone tries this, just make a 0 writer.
+	if n < 0 {
+		n = 0
+	}
+	return &LimitWriter{
+		Limit: n,
+		N:     0,
+		W:     w,
+	}
+}
+
+func (l *LimitWriter) Write(p []byte) (int, error) {
+	if l.N >= l.Limit {
+		return 0, ErrLimitReached
+	}
+
+	// Write 0 bytes if the limit is to be exceeded.
+	if int64(len(p)) > l.Limit-l.N {
+		return 0, ErrLimitReached
+	}
+
+	n, err := l.W.Write(p)
+	l.N += int64(n)
+	return n, err
+}