misc: block: streamline #[source] and Error

This streamlines the code base to follow best practices for
error handling in Rust: Each error struct implements
std::error::Error (most due via thiserror::Error derive macro)
and sets its source accordingly.

This allows future work that nicely prints the error chains,
for example.

So far, the convention is that each error prints its
sub error as part of its Display::fmt() impl.

Signed-off-by: Philipp Schuster <philipp.schuster@cyberus-technology.de>
On-behalf-of: SAP philipp.schuster@sap.com

block/src/lib.rs

@@ -68,9 +68,9 @@ pub const SECTOR_SIZE: u64 = 0x01 << SECTOR_SHIFT;
 #[derive(Error, Debug)]
 pub enum Error {
     #[error("Guest gave us bad memory addresses")]
-    GuestMemory(GuestMemoryError),
+    GuestMemory(#[source] GuestMemoryError),
     #[error("Guest gave us offsets that would have overflowed a usize")]
-    CheckedOffset(GuestAddress, usize),
+    CheckedOffset(GuestAddress, usize /* sector offset */),
     #[error("Guest gave us a write only descriptor that protocol says to read from")]
     UnexpectedWriteOnlyDescriptor,
     #[error("Guest gave us a read only descriptor that protocol says to write to")]
@@ -80,21 +80,21 @@ pub enum Error {
     #[error("Guest gave us a descriptor that was too short to use")]
     DescriptorLengthTooSmall,
     #[error("Failed to detect image type: {0}")]
-    DetectImageType(std::io::Error),
+    DetectImageType(#[source] std::io::Error),
     #[error("Failure in fixed vhd: {0}")]
-    FixedVhdError(std::io::Error),
+    FixedVhdError(#[source] std::io::Error),
     #[error("Getting a block's metadata fails for any reason")]
     GetFileMetadata,
     #[error("The requested operation would cause a seek beyond disk end")]
     InvalidOffset,
     #[error("Failure in qcow: {0}")]
-    QcowError(qcow::Error),
+    QcowError(#[source] qcow::Error),
     #[error("Failure in raw file: {0}")]
-    RawFileError(std::io::Error),
+    RawFileError(#[source] std::io::Error),
     #[error("The requested operation does not support multiple descriptors")]
     TooManyDescriptors,
     #[error("Failure in vhdx: {0}")]
-    VhdxError(VhdxError),
+    VhdxError(#[source] VhdxError),
 }
 
 fn build_device_id(disk_path: &Path) -> result::Result<String, Error> {
@@ -132,33 +132,33 @@ pub fn build_serial(disk_path: &Path) -> Vec<u8> {
 #[derive(Error, Debug)]
 pub enum ExecuteError {
     #[error("Bad request: {0}")]
-    BadRequest(Error),
+    BadRequest(#[source] Error),
     #[error("Failed to flush: {0}")]
-    Flush(io::Error),
+    Flush(#[source] io::Error),
     #[error("Failed to read: {0}")]
-    Read(GuestMemoryError),
+    Read(#[source] GuestMemoryError),
     #[error("Failed to read_exact: {0}")]
-    ReadExact(io::Error),
+    ReadExact(#[source] io::Error),
     #[error("Failed to seek: {0}")]
-    Seek(io::Error),
+    Seek(#[source] io::Error),
     #[error("Failed to write: {0}")]
-    Write(GuestMemoryError),
+    Write(#[source] GuestMemoryError),
     #[error("Failed to write_all: {0}")]
-    WriteAll(io::Error),
+    WriteAll(#[source] io::Error),
     #[error("Unsupported request: {0}")]
     Unsupported(u32),
     #[error("Failed to submit io uring: {0}")]
-    SubmitIoUring(io::Error),
+    SubmitIoUring(#[source] io::Error),
     #[error("Failed to get guest address: {0}")]
-    GetHostAddress(GuestMemoryError),
+    GetHostAddress(#[source] GuestMemoryError),
     #[error("Failed to async read: {0}")]
-    AsyncRead(AsyncIoError),
+    AsyncRead(#[source] AsyncIoError),
     #[error("Failed to async write: {0}")]
-    AsyncWrite(AsyncIoError),
+    AsyncWrite(#[source] AsyncIoError),
     #[error("failed to async flush: {0}")]
-    AsyncFlush(AsyncIoError),
+    AsyncFlush(#[source] AsyncIoError),
     #[error("Failed allocating a temporary buffer: {0}")]
-    TemporaryBufferAllocation(io::Error),
+    TemporaryBufferAllocation(#[source] io::Error),
 }
 
 impl ExecuteError {