Handle ipfs object diff error

[schomatis]

https://github.com/ipfs/go-ipfs/blob/e4e35047bd338029fd2c4ff25c71bc6cccc35bf3/core/commands/object/diff.go#L76-L95

The error generated by the Diff() function is being ignored and we're silently failing here, e.g.,

IPFS=$(echo "IPFS" | ipfs add --raw-leaves --chunker size-1 -Q)
IPFA=$(echo "IPFA" | ipfs add --raw-leaves --chunker size-1 -Q)
ipfs object diff $IPFS $IPFA
# It prints nothing, giving the impression the objects are the same (which they are not).
# It should print "ERROR: expected protobuf dag node".

[schomatis]

@overbool Wanna take this one?

[schomatis]

BTW, one thing I'm not understanding about Go: it doesn't complain about the unused err variable even though it is being redefined, so it is in fact (from my understanding) a new variable. If I rename it to newErr it does complain, but redefining a new variable with an already used name (err) seems fine even if it's not used. @Stebalien What am I missing?

[overbool]

@schomatis I am willing to do it, thx.

[schomatis]

BTW, one thing I'm not understanding about Go: it doesn't complain about the unused err variable even though it is being redefined, so it is in fact (from my understanding) a new variable. If I rename it to newErr it does complain, but redefining a new variable with an already used name (err) seems fine even if it's not used. @Stebalien What am I missing?

Answering my own question: from https://golang.org/ref/spec#Short_variable_declarations

Unlike regular variable declarations, a short variable declaration may redeclare variables provided they were originally declared earlier in the same block (or the parameter lists if the block is the function body) with the same type, and at least one of the non-blank variables is new. As a consequence, redeclaration can only appear in a multi-variable short declaration. Redeclaration does not introduce a new variable; it just assigns a new value to the original.

So the err := in the highlighted code is not a new variable, is just an assignment of the old one ("redeclaration" seems to be just an alias of "assign value"), which can be a bit confusing IMO.