Add default small delay to retry.

50ms delay on first retry.
250ms delay on 2nd retry.

This at least gives other tasks a chance to run.

If retry n is set higher, the delay increases to 1250ms, 6250ms ...
max_delay caps the dealy at 10s by default.

This should handle network-timescale issues without creating undue load.

base/error.jl

@@ -52,7 +52,7 @@ end
 
 
 """
-    retry(f, [condition], n=3)
+    retry(f, [condition], n=3; max_delay=10)
 
 Returns a lambda that retries function `f` up to `n` times in the
 event of an exception. If `condition` is a `Type` then retry only
@@ -61,8 +61,9 @@ for exceptions of that type. If `condition` is a function
 
 e.g. `retry(http_get, e->e.status == "503")(url)` or `retry(read, UVError)(io)`.
 """
-function retry(f::Function, condition::Function=e->true, n::Int=3)
+function retry(f::Function, condition::Function=e->true, n::Int=3; max_delay=10)
     (args...) -> begin
+        delay = 0.05
         for i = 1:n
             try
                 return f(args...)
@@ -71,6 +72,8 @@ function retry(f::Function, condition::Function=e->true, n::Int=3)
                     rethrow(e)
                 end
             end
+            sleep(delay * (0.8 + (rand() * 0.4)))
+            delay = min(max_delay, delay * 5)
         end
     end
 end

doc/stdlib/base.rst

@@ -1218,7 +1218,7 @@ Errors
 
    An error occurred when running a module's ``__init__`` function. The actual error thrown is available in the ``.error`` field.
 
-.. function:: retry(f, [condition], n=3)
+.. function:: retry(f, [condition], n=3; max_delay=10)
 
    .. Docstring generated from Julia source