feat(bls): boot counting (#107)

* feat(bls): parse and render boot counters

* feat(bls): sort entries with no boot tries left last

* feat(boot): strip boot counters from bls entry ids

* feat(boot): consume bls boot counter tries before booting

* fix(boot): never default to a bad boot counter entry while a good one exists

crates/boot/src/boot_counter.rs

@@ -0,0 +1,54 @@
+use alloc::{format, string::String};
+use anyhow::{Context, Result};
+use edera_sprout_bls::BootCounter;
+use uefi::{
+    CString16, Handle,
+    fs::{FileSystem, PathBuf},
+    proto::media::fs::SimpleFileSystem,
+};
+
+/// Where the boot counter of an entry is stored, so that a try can be consumed when the
+/// entry is booted.
+#[derive(Clone)]
+pub struct BootCounterTarget {
+    /// The counter as it is currently encoded in the file name.
+    pub counter: BootCounter,
+    /// The handle of the filesystem that holds the entry file.
+    pub filesystem: Handle,
+    /// The directory that holds the entry file.
+    pub directory: PathBuf,
+    /// The entry id, which is the file name without the counter and extension.
+    pub id: String,
+    /// The current file name, including the counter and the extension.
+    pub file_name: String,
+}
+
+impl BootCounterTarget {
+    /// Consumes one try by renaming the entry file, such as `foo+3.conf` to `foo+2-1.conf`.
+    /// Returns the path of the renamed file.
+    pub fn consume(&self) -> Result<PathBuf> {
+        // The extension is taken from the current file name, as it may be in any case.
+        let extension = &self.file_name[self.file_name.len() - ".conf".len()..];
+        let new_name = format!(
+            "{}{}",
+            self.counter.decremented().render(&self.id),
+            extension
+        );
+
+        let mut from = self.directory.clone();
+        from.push(PathBuf::from(
+            CString16::try_from(self.file_name.as_str()).context("invalid entry file name")?,
+        ));
+        let mut to = self.directory.clone();
+        to.push(PathBuf::from(
+            CString16::try_from(new_name.as_str()).context("invalid new entry file name")?,
+        ));
+
+        let fs = uefi::boot::open_protocol_exclusive::<SimpleFileSystem>(self.filesystem)
+            .context("unable to open the entry filesystem")?;
+        let mut fs = FileSystem::new(fs);
+        fs.rename(&from, &to)
+            .context("unable to rename the entry file")?;
+        Ok(to)
+    }
+}