std.sha1 secureHashFile memory usage

[ilitirit]

I'm comparing the performance of various hashing modules and it appears that 
`secureHashFile` has memory leaks. Or am I simply using it incorrectly?

Here's a comparison between nimSHA2 and std/sha1:
    
    
    # Compiled on a Windows 10 machine with:
    # nim c -d:nimTypeNames sha.nim
    
    # Nim Compiler Version 1.2.0 [Windows: amd64]
    
    import std/sha1
    import nimSHA2
    
    proc sha256(fileName : string) : string =
        const blockSize: int = 1024 * 1024
        var file : File
        var sha = initSHA[SHA256]()
        
        try:
          file = open(filename, FileMode.fmRead, blockSize)
          var buffer = newString(blockSize)
          var bytesRead = file.readChars(buffer, 0, blockSize)
          while bytesRead > 0:
            sha.update(buffer.substr(0, bytesRead - 1))
            bytesRead = file.readChars(buffer, 0, blockSize)
        finally:
            if file != nil:
              close(file)
        
        sha.final().toHex
    
    echo "Calculating SHA256"
    echo sha256("Fedora-Workstation-Live-x86_64-32-1.6.iso") # 1.9GB file
    
    dumpNumberOfInstances()
    
    echo "Calculating SHA1"
    echo $secureHashFile("Fedora-Workstation-Live-x86_64-32-1.6.iso")
    
    dumpNumberOfInstances()
    
    echo "Press enter to exit"
    discard stdin.readLine()
    
    
    Run

Output: 
    
    
    Calculating SHA256
    4D0F6653E2E0860C99FFE0EF274A46D875FB85BD2A40CB896DCE1ED013566924
    [Heap] string: #40; bytes: 2197217
    [Heap] WideCStringObj: #0; bytes: 880
    [Heap] total number of bytes: 2198097
    [Heap] allocs/deallocs: 1921/1876
    Calculating SHA1
    A936C00AB4BF1B28963823A8E80A828D30812717
    [Heap] string: #26; bytes: 1966179519
    [Heap] WideCStringObj: #0; bytes: 1760
    [Heap] total number of bytes: 1966181279
    [Heap] allocs/deallocs: 1951/1918
    Press enter to exit
    
    Run

Calling them separately results in a post-computation RAM usage of less than 
10MB with SHA256 and 1.8GB with SHA1. Calling `secureHashFile` on large files 
from a Win32 front-end also results in the type of text corruption that's 
usually indicative of a memory leak somewhere.