@bboreham can you check if this can fix your issue? I tryed on local

docker load -i busybox.tar
28779c628bf0: Loading layer [==================================================>] 1.311 MB/1.311 MB
0292957ea1dd: Loading layer [==================================================>] 1.024 kB/1.024 kB
The image busybox:latest already exists, renaming the old one with ID sha256:47bcc53f74dc94b1920f0b34f6036096526296767650f223433fe65c35f149eb to empty string

I don't want to seem ungrateful, but are you sure that the correct course of action is to edit the one place where this one message is generated? Surely it needs a more general mechanism by which messages may be output in the appropriate context and fed through to the other end.

I'm not set up to build Docker right now so it will take me a while to be able to test it.

@bboreham You concern is reasonable, I have already checked, this is the only place where it will write message to outStream

@bboreham I update the fix to address your concern -:)
I think this's better

@coolljt0725 i wonder if we could add a test somehow (unit test?)

@tiborvass I'll try

I made an easier repro - #21957 (comment)

Building docker from your branch coolljt0725:fix_21957 works fine with matched client and daemon: strace shows it sending valid JSON:

HTTP/1.1 200 OK\r\nContent-Type: application/json\r\nServer: Docker/1.11.0-dev (linux)\r\nDate: Tue, 12 Apr 2016 14:11:55 GMT\r\nContent-Length: 171\r\n\r\n{\"stream\":\"The image busybox:xxx already exists, renaming the old one with ID ...

but when called from a 1.10.3 client I get:

$ /usr/bin/docker-1.10.3 load -i xxx
invalid character 'T' looking for beginning of value

strace shows:

HTTP/1.1 200 OK\r\nContent-Type: application/json\r\nServer: Docker/1.11.0-dev (linux)\r\nDate: Tue, 12 Apr 2016 14:17:05 GMT\r\nContent-Length: 155\r\n\r\nThe image busybox:xxx already exists, renaming the old one with ID sha256:d34ea343a882c1f8ad2692872d0a3db95cccd0d3fbdfeee015113871b4f171b9 to empty string\n

@bboreham really thank you for your testing :-)
the fix is updated, it should fix unmatch client and server
@tiborvass I added a intergration-cli

I tested 4cc43fdbcafbaab3c3ef98293fa3d70393805078 - doesn't look too different from the current.

@bboreham the commit you tested is old, I just updated the fix

OK, I tested bbd0816d674792501072b795fb8e8afde04192c4, and it cures the error message.

Daemon is still sending an empty payload on re-load as noted at #21957 (comment)

LGTM

@tiborvass I made a update, make the the content-type as application/json only if user want the load progressbar, so this will not break old API calls. sorry, you have to review it again :)

Re LGTM

LGTM