How do I go from io.ReadCloser to io.ReadSeeker? -

i'm trying download file s3 , upload file bucket in s3. copy api won't work here because i've been told not use it.

getting object s3 has response.body that's io.readcloser , upload file, payload takes body that's io.readseeker.

the way can figure out saving response.body file passing file io.readseeker. require writing entire file disk first reading entire file disk sounds pretty wrong.

what is:

resp, _ := conn.getobject(&s3.getobjectinput{key: "bla"}) conn.putobject(&s3.putobjectinput{body: resp.body}) // resp.body io.readcloser , field type expects io.readseeker 

question is, how go io.readcloser io.readseeker in efficient way possible?

io.readseeker interface groups basic read() , seek() methods. definition of seek() method:

seek(offset int64, whence int) (int64, error) 

an implementation of seek() method requires able seek anywhere in source, requires all source available or reproducible. file perfect example, file saved permanently disk , part of can read @ time.

response.body implemented read underlying tcp connection. reading underlying tcp connection gives data client @ other side sends you. data not cached, , client won't send data again upon request. that's why response.body not implement io.seeker (and io.readseeker either).

so in order obtain io.readseeker io.reader or io.readcloser, need caches all data, upon request can seek anywhere in that.

this caching mechanism may writing file mentioned, or can read memory, []byte using ioutil.readall(), , can use bytes.newreader() obtain io.readseeker []byte. of course has limitations: content must fit memory, , might not want reserve amount of memory file copy operation.

all in all, implementation of io.seeker or io.readseeker requires source data available, best bet writing file, or small files reading []byte , streaming content of byte slice.