Relax "extract_pin_from_path" logic

The logic of extract_pin_from_path is too rigid, expecting GPIOs to only
exist at the "/sys/class/gpio" path. The problem with this is that the
kernel may actually symlink the directory in "/sys/class/gpio" to a chip
or SOC-specific path, and the logic prior to extracting the pin from the
path performs a canonicalization, breaking the assumption.

So, for example, the following is the output of a gpio on the board I'm
working with:

    root# readlink /var/run/gpio/hwid-0
        /sys/class/gpio/gpio122

    root# readlink /sys/class/gpio/gpio122
        ../../devices/soc0/soc/2000000.aips-bus/20a8000.gpio/gpiochip3/gpio/gpio122

This commit fixes the issue by relaxing the restrictions of
extract_pin_from_path to only look for a final file component that looks
like "gpioXXXX".

Signed-off-by: Nick Stevens <nick@bitcurry.com>

Author: nastevens

CHANGELOG.md

@@ -2,6 +2,12 @@
 
 ## [master] - Unreleased
 
+### Fixed
+
+- Relaxed path restrictions on `Pin::from_path` to allow for directories
+  outside of `/sys/class/gpio`, required for some SOCs that symlink outside of
+  that directory.
+
 ## [0.5.2] - 2018-03-02
 
 ### Changed

src/lib.rs

@@ -77,8 +77,6 @@ use tokio_core::reactor::{Handle, PollEvented};
 mod error;
 pub use error::Error;
 
-const GPIO_PATH_PREFIX: &'static str = "/sys/class/gpio/gpio";
-
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub struct Pin {
     pin_num: u64,
@@ -177,21 +175,17 @@ impl Pin {
                                           a directory"
                                                  .to_owned()));
         }
-        let num = Pin::extract_pin_from_path(&pb.to_str().unwrap_or(""))?;
+        let num = Pin::extract_pin_from_path(&pb)?;
         Ok(Pin::new(num))
     }
 
     /// Extract pin number from paths like /sys/class/gpio/gpioXXX
-    fn extract_pin_from_path(path: &str) -> Result<u64> {
-        if path.starts_with(GPIO_PATH_PREFIX) {
-            path.split_at(GPIO_PATH_PREFIX.len()).1.parse::<u64>().or(
-                Err(
-                    Error::InvalidPath(format!("{:?}", path)),
-                ),
-            )
-        } else {
-            Err(Error::InvalidPath(format!("{:?}", path)))
-        }
+    fn extract_pin_from_path<P: AsRef<Path>>(path: P) -> Result<u64> {
+        path.as_ref()
+            .file_name()
+            .and_then(|filename| filename.to_str())
+            .and_then(|filename_str| filename_str.trim_left_matches("gpio").parse::<u64>().ok())
+            .ok_or(Error::InvalidPath(format!("{:?}", path.as_ref())))
     }
 
     /// Get the pin number
@@ -483,14 +477,16 @@ impl Pin {
 
 #[test]
 fn extract_pin_fom_path_test() {
-    let tok = Pin::extract_pin_from_path(&"/sys/class/gpio/gpio951");
-    assert_eq!(951, tok.unwrap());
-    let err1 = Pin::extract_pin_from_path(&"/sys/is/error/gpio/gpio111");
+    let tok1 = Pin::extract_pin_from_path(&"/sys/class/gpio/gpio951");
+    assert_eq!(951, tok1.unwrap());
+    let tok2 = Pin::extract_pin_from_path(&"/sys/CLASS/gpio/gpio951/");
+    assert_eq!(951, tok2.unwrap());
+    let tok3 = Pin::extract_pin_from_path(&"../../devices/soc0/gpiochip3/gpio/gpio124");
+    assert_eq!(124, tok3.unwrap());
+    let err1 = Pin::extract_pin_from_path(&"/sys/CLASS/gpio/gpio");
     assert_eq!(true, err1.is_err());
-    let err2 = Pin::extract_pin_from_path(&"/sys/CLASS/gpio/gpio");
+    let err2 = Pin::extract_pin_from_path(&"/sys/class/gpio/gpioSDS");
     assert_eq!(true, err2.is_err());
-    let err3 = Pin::extract_pin_from_path(&"/sys/class/gpio/gpioSDS");
-    assert_eq!(true, err3.is_err());
 }
 
 #[derive(Debug)]