// GetBlock attempts to retrieve a particular block from peers within the
// deadline enforced by the context.
func (bs *Bitswap) GetBlock(parent context.Context, k *cid.Cid) (blocks.Block, error) {
if k == nil {
log.Error("nil cid in GetBlock")
return nil, blockstore.ErrNotFound
}
// Any async work initiated by this function must end when this function
// returns. To ensure this, derive a new context. Note that it is okay to
// listen on parent in this scope, but NOT okay to pass |parent| to
// functions called by this one. Otherwise those functions won't return
// when this context's cancel func is executed. This is difficult to
// enforce. May this comment keep you safe.
ctx, cancelFunc := context.WithCancel(parent)
ctx = logging.ContextWithLoggable(ctx, loggables.Uuid("GetBlockRequest"))
log.Event(ctx, "Bitswap.GetBlockRequest.Start", k)
defer log.Event(ctx, "Bitswap.GetBlockRequest.End", k)
defer func() {
cancelFunc()
}()
promise, err := bs.GetBlocks(ctx, []*cid.Cid{k})
if err != nil {
return nil, err
}
select {
case block, ok := <-promise:
if !ok {
select {
case <-ctx.Done():
return nil, ctx.Err()
default:
return nil, errors.New("promise channel was closed")
}
}
return block, nil
case <-parent.Done():
return nil, parent.Err()
}
}
// main roadmap:
// - parse the commandline to get a cmdInvocation
// - if user requests help, print it and exit.
// - run the command invocation
// - output the response
// - if anything fails, print error, maybe with help
func main() {
rand.Seed(time.Now().UnixNano())
ctx := logging.ContextWithLoggable(context.Background(), loggables.Uuid("session"))
var err error
var invoc cmdInvocation
defer invoc.close()
// we'll call this local helper to output errors.
// this is so we control how to print errors in one place.
printErr := func(err error) {
fmt.Fprintf(os.Stderr, "Error: %s\n", err.Error())
}
stopFunc, err := profileIfEnabled()
if err != nil {
printErr(err)
os.Exit(1)
}
defer stopFunc() // to be executed as late as possible
// this is a local helper to print out help text.
// there's some considerations that this makes easier.
printHelp := func(long bool, w io.Writer) {
helpFunc := cmdsCli.ShortHelp
if long {
helpFunc = cmdsCli.LongHelp
}
helpFunc("ipfs", Root, invoc.path, w)
}
// this is a message to tell the user how to get the help text
printMetaHelp := func(w io.Writer) {
cmdPath := strings.Join(invoc.path, " ")
fmt.Fprintf(w, "Use 'ipfs %s --help' for information about this command\n", cmdPath)
}
// Handle `ipfs help'
if len(os.Args) == 2 {
if os.Args[1] == "help" {
printHelp(false, os.Stdout)
os.Exit(0)
} else if os.Args[1] == "--version" {
os.Args[1] = "version"
}
}
// parse the commandline into a command invocation
parseErr := invoc.Parse(ctx, os.Args[1:])
// BEFORE handling the parse error, if we have enough information
// AND the user requested help, print it out and exit
if invoc.req != nil {
longH, shortH, err := invoc.requestedHelp()
if err != nil {
printErr(err)
os.Exit(1)
}
if longH || shortH {
printHelp(longH, os.Stdout)
os.Exit(0)
}
}
// ok now handle parse error (which means cli input was wrong,
// e.g. incorrect number of args, or nonexistent subcommand)
if parseErr != nil {
printErr(parseErr)
// this was a user error, print help.
if invoc.cmd != nil {
// we need a newline space.
fmt.Fprintf(os.Stderr, "\n")
printHelp(false, os.Stderr)
}
os.Exit(1)
}
// here we handle the cases where
// - commands with no Run func are invoked directly.
// - the main command is invoked.
if invoc.cmd == nil || invoc.cmd.Run == nil {
printHelp(false, os.Stdout)
os.Exit(0)
}
// ok, finally, run the command invocation.
intrh, ctx := invoc.SetupInterruptHandler(ctx)
defer intrh.Close()
output, err := invoc.Run(ctx)
if err != nil {
printErr(err)
// if this error was a client error, print short help too.
if isClientError(err) {
printMetaHelp(os.Stderr)
}
os.Exit(1)
}
// everything went better than expected :)
_, err = io.Copy(os.Stdout, output)
if err != nil {
printErr(err)
os.Exit(1)
}
}