fmt: Scanning '%%' is a verb

[pavel]

What version of Go are you using (go version)?

$ go version
go version go1.13 linux/amd64

Does this issue reproduce with the latest release?

Yes

What did you do?

https://play.golang.org/p/PNBiGNussAs

package main

import (
	"fmt"
	"log"
)

func readVersion(f, s string) {
	var major, minor uint8

	_, err := fmt.Sscanf(s, f, &major, &minor)
	if err != nil {
		log.Println("Failed to parse", s, ". Reason:", err)
		return
	}

	log.Println("Parsed", s, "into", major, minor)
}

func main() {
	readVersion("%%VER-%d.%d", "%VER-1.4")
	readVersion("%%VER-%d.%d", "%ER-1.4") // fails: "bad verb '%%' for integer"
	readVersion("VER-%d.%d", "VER-1.4")
	readVersion("VER-%d.%d", "ER-1.4") // fails: "input does not match format"
}

What did you expect to see?

Second run should return input does not match format, just like the call number 4.

What did you see instead?

bad verb '%%' for integer error.

[rusq]

Observing the same behaviour for Go 1.8, Go 1.9.5 and Go 1.10.5, not something introduced in 1.13.

34180.go contains the code from the playground mentioned above. https://play.golang.org/p/PNBiGNussAs

$ go1.8 run 34180.go
2019/09/10 22:10:03 Parsed %VER-1.4 into 1 4
2019/09/10 22:10:03 Failed to parse %ER-1.4 . Reason: bad verb '%%' for integer
2019/09/10 22:10:03 Parsed VER-1.4 into 1 4
2019/09/10 22:10:03 Failed to parse ER-1.4 . Reason: input does not match format

[pavel]

The logic fails here:

if format[i] != '%' {

This assumes that a scanning problem is signaled when i points to a non % rune, which is not true.
During a line 2 call in the original example, advance function fails to match runes E and V returning -1. At this point state is i = 0, w = -1 and format[i] = '%'. This causes scanning to continue, failing later at integer scanning.

CC @smasher164 Investigation done.

[pavel]

Another failing case would be https://play.golang.org/p/itkheUx5t3O

package main

import (
	"fmt"
	"log"
)

func main() {
	var n int
	if _, err := fmt.Sscanf("123 %", "%d%%", &n); err != nil {
		log.Println(err)
	} else {
		log.Println("OK", n)
	}
}

Error returned too few operands for format '%%'.
Expected error to be input does not match format.

[smasher164]

/cc @robpike @martisch

[gopherbot]

Change https://golang.org/cl/202879 mentions this issue: fmt: fix handling of %% verb in Scanf